diff --git a/code_obj.lua b/code_obj.lua
index d3b9260..7fa91fc 100644
--- a/code_obj.lua
+++ b/code_obj.lua
@@ -173,11 +173,11 @@ do
     __init = function(self, source, ...)
       self.source = source
       self.bits, self.indents, self.current_indent = { }, { }, 0
-      self:append(...)
       if type(self.source) == 'string' then
         self.source = Source:from_string(self.source)
       end
-      return assert(self.source and Source:is_instance(self.source))
+      assert(self.source and Source:is_instance(self.source))
+      return self:append(...)
     end,
     __base = _base_0,
     __name = "Code"
diff --git a/code_obj.moon b/code_obj.moon
index ef19ed4..74e9a15 100644
--- a/code_obj.moon
+++ b/code_obj.moon
@@ -46,10 +46,10 @@ class Source
 class Code
     new: (@source, ...)=>
         @bits, @indents, @current_indent = {}, {}, 0
-        @append(...)
         if type(@source) == 'string'
             @source = Source\from_string(@source)
         assert(@source and Source\is_instance(@source))
+        @append(...)
             
     append: (...)=>
         n = select("#",...)
diff --git a/core/collections.nom b/core/collections.nom
index 0f53679..48a4330 100644
--- a/core/collections.nom
+++ b/core/collections.nom
@@ -9,6 +9,7 @@ use "core/operators.nom"
 # List/dict functions:
 
 # Indexing
+test: assume: (2nd to last in [1,2,3,4,5]) is 4
 compile [..]
     %index st to last in %list, %index nd to last in %list, %index rd to last in %list
     %index th to last in %list
@@ -20,11 +21,13 @@ parse [last in %list] as: 1st to last in %list
 parse [first in %list] as: %list.1
 
 # Membership testing
+test: assume: 3 is in [1,2,3,4,5]
 action [%item is in %list, %list contains %item, %list has %item]
     for %key = %value in %list
         if (%key is %item): return (yes)
     return (no)
 
+test: assume: 99 isn't in [1,2,3]
 action [..]
     %item isn't in %list, %item is not in %list
     %list doesn't contain %item, %list does not contain %item
@@ -34,9 +37,13 @@ action [..]
         if (%key is %item): return (no)
     return (yes)
 
+test: assume: {x:no} has key "x"
 parse [%list has key %index, %list has index %index] as
     %list.%index != (nil)
 
+test
+    assume: {x:no} doesn't have key "y"
+    assume: not: {x:no} doesn't have key "x"
 parse [..]
     %list doesn't have key %index, %list does not have key %index
     %list doesn't have index %index, %list does not have index %index
@@ -46,6 +53,15 @@ parse [..]
 compile [number of keys in %list] to
     Lua value "utils.size(\(%list as lua expr))"
 
+test
+    %list <- [1,2,3,4,5]
+    append 6 to %list
+    assume: (last in %list) is 6
+    pop from %list
+    assume: (last in %list) is 5
+    remove index 1 from %list
+    assume: (first in %list) is 2
+
 compile [append %item to %list, add %item to %list, to %list add %item, to %list append %item] to
     Lua "table.insert(\(%list as lua expr), \(%item as lua expr))"
 
@@ -58,6 +74,7 @@ compile [remove index %index from %list] to
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 # List Comprehension
+test: assume: ((% * %) for % in [1,2,3]) = [1,4,9]
 parse [%expression for %item in %iterable] as
     result of
         %comprehension <- []
@@ -76,9 +93,11 @@ parse [..]
         return %comprehension
 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+test: assume: ((% * %) for % in 1 to 3) = [1,4,9]
 parse [%expression for %var in %start to %stop] as
     %expression for %var in %start to %stop via 1
 
+test: assume: ("\%k,\%v" for %k = %v in {x:1}) = ["x,1"]
 parse [..]
     %expression for %key = %value in %iterable
     %expression for (%key,%value) in %iterable
@@ -90,6 +109,7 @@ parse [..]
         return %comprehension
 
 # Dict comprehensions
+test: assume: ((% * %) = % for % in [1,2,3]) = {1:1,4:2,9:3}
 parse [..]
     %key = %value for %item in %iterable
     (%key,%value) for %item in %iterable
@@ -100,6 +120,7 @@ parse [..]
             %comprehension.%key <- %value
         return %comprehension
 
+test: assume: (%k = (%v * %v) for %k = %v in {x:1,y:2,z:3}) = {x:1,y:4,z:9}
 parse [..]
     %key = %value for %src_key = %src_value in %iterable
     (%key,%value) for (%src_key,%src_value) in %iterable
@@ -110,7 +131,23 @@ parse [..]
             %comprehension.%key <- %value
         return %comprehension
 
+parse [..]
+    %key = %value for %item in %start to %stop via %step
+    (%key,%value) for %item in %start to %stop via %step
+..as
+    result of
+        %comprehension <- {}
+        for %item in %start to %stop via %step
+            %comprehension.%key <- %value
+        return %comprehension
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+test: assume: ((% * %) = % for % in 1 to 3) = {1:1,4:2,9:3}
+parse [..]
+    %key = %value for %item in %start to %stop
+    (%key,%value) for %item in %start to %stop
+..as: %key = %value for %item in %start to %stop via 1
+
+test: assume: ([[1,2],[3,4]] flattened) = [1,2,3,4]
 action [%lists flattened]
     %flat <- []
     for %list in %lists
@@ -118,14 +155,24 @@ action [%lists flattened]
             add %item to %flat
     return %flat
 
+test: assume: (entries in {x:1}) = [{key:"x",value:1}]
 parse [entries in %dict] as: {key:%k, value:%v} for %k = %v in %dict
+
+test: assume: (keys in {x:1}) = ["x"]
 parse [keys in %dict, keys of %dict] as: %k for %k = %v in %dict
+
+test: assume: (values in {x:1}) = [1]
 parse [values in %dict, values of %dict] as: %v for %k = %v in %dict
 
 # Metatable stuff
+test
+    %t <- {}
+    set %t's metatable to {__tostring:[%]->"XXX"}
+    assume: "\%t" = "XXX"
 compile [set %dict's metatable to %metatable] to
     Lua "setmetatable(\(%dict as lua expr), \(%metatable as lua expr));"
 
+test: assume: ({} with fallback % -> (% + 1)).10 = 11
 compile [%dict with fallback %key -> %value] to
     Lua value ".."
         setmetatable(\(%dict as lua expr), {__index=function(self, \(%key as lua expr))
@@ -134,6 +181,8 @@ compile [%dict with fallback %key -> %value] to
             return value
         end})
 
+test: assume: 1 is 2
+
 # Sorting
 compile [sort %items] to: Lua "table.sort(\(%items as lua expr));"
 parse [..]
diff --git a/core/errors.nom b/core/errors.nom
index 812052e..91f9b4f 100644
--- a/core/errors.nom
+++ b/core/errors.nom
@@ -8,17 +8,17 @@ compile [traceback %] to: Lua value "debug.traceback('', \(% as lua expr))"
 compile [barf] to: Lua "error(nil, 0);"
 compile [barf %msg] to: Lua "error(\(%msg as lua expr), 0);"
 compile [assume %condition] to
-    lua> "local \%assumption = 'Assumption failed: '..tostring(nomsu:tree_to_nomsu(\%condition));"
+    lua> "local \%assumption = 'Assumption failed: '..tostring(nomsu:tree_to_nomsu(\%condition))"
     return
         Lua ".."
             if not \(%condition as lua expr) then
-                error(\(quote "\%assumption"), 0);
+                error(\(quote "\%assumption"), 0)
             end
 
 compile [assume %condition or barf %message] to
     Lua ".."
         if not \(%condition as lua expr) then
-            error(\(%message as lua expr), 0);
+            error(\(%message as lua expr), 0)
         end
 
 # Try/except
diff --git a/core/metaprogramming.nom b/core/metaprogramming.nom
index 242db04..5a80c64 100644
--- a/core/metaprogramming.nom
+++ b/core/metaprogramming.nom
@@ -124,8 +124,13 @@ compile [parse %actions as %body] to
         return ret
 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-compile [%tree as lua expr] to
-    Lua value "nomsu:compile(\(=lua "nomsu:compile(\%tree):as_expr()")):as_expr()"
+lua> ".."
+    COMPILE_ACTIONS["% as lua expr"] = function(nomsu, tree, _t)
+        return LuaCode.Value(tree.source, "nomsu:compile(", nomsu:compile(_t):as_expr(), "):as_expr()")
+    end
+#
+    compile [%tree as lua expr] to
+        Lua value "nomsu:compile(\(=lua "nomsu:compile(\%tree):as_expr()")):as_expr()"
 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 compile [%tree as lua] to
diff --git a/core/operators.nom b/core/operators.nom
index d434ea7..0712ada 100644
--- a/core/operators.nom
+++ b/core/operators.nom
@@ -163,6 +163,7 @@ compile [%x ARSHIFT %shift, %x >> %shift] to: Lua value "(\(%x as lua expr) >> \
 # Unary operators
 compile [- %] to: Lua value "(- \(% as lua expr))"
 compile [not %] to: Lua value "(not \(% as lua expr))"
+test: assume: (length of [1,2,3]) = 3
 compile [length of %list, || %list ||] to: Lua value "(#\(%list as lua expr))"
 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/error_handling.lua b/error_handling.lua
index b0e6dde..d88cfeb 100644
--- a/error_handling.lua
+++ b/error_handling.lua
@@ -45,25 +45,31 @@ debug.getinfo = function(thread, f, what)
           info.lastlinedefined = assert(map[info.lastlinedefined])
         end
         info.short_src = info.source:match('@([^[]*)') or info.short_src
+        if info.name then
+          do
+            local tmp = info.name:match("^A_([a-zA-Z0-9_]*)$")
+            if tmp then
+              info.name = tmp:gsub("_", " "):gsub("x([0-9A-F][0-9A-F])", function(self)
+                return string.char(tonumber(self, 16))
+              end)
+            else
+              info.name = info.name
+            end
+          end
+        else
+          info.name = "main chunk"
+        end
       end
     end
   end
   return info
 end
 local print_error
-print_error = function(error_message, stack_offset)
-  if stack_offset == nil then
-    stack_offset = 3
-  end
+print_error = function(error_message, start_fn, stop_fn)
   io.stderr:write(tostring(colored.red("ERROR:")) .. " " .. tostring(colored.bright(colored.red((error_message or "")))) .. "\n")
   io.stderr:write("stack traceback:\n")
-  local get_line
-  get_line = function(file, line_no)
-    local start = LINE_STARTS[file][line_no] or 1
-    local stop = (LINE_STARTS[file][line_no + 1] or 0) - 1
-    return file:sub(start, stop)
-  end
-  local level = stack_offset
+  local level = 1
+  local found_start = false
   while true do
     local _continue_0 = false
     repeat
@@ -71,10 +77,14 @@ print_error = function(error_message, stack_offset)
       if not calling_fn then
         break
       end
-      if calling_fn.func == run then
+      level = level + 1
+      if not (found_start) then
+        if calling_fn.func == start_fn then
+          found_start = true
+        end
+        _continue_0 = true
         break
       end
-      level = level + 1
       local name = calling_fn.name and "function '" .. tostring(calling_fn.name) .. "'" or nil
       if calling_fn.linedefined == 0 then
         name = "main chunk"
@@ -98,8 +108,8 @@ print_error = function(error_message, stack_offset)
           end
           local filename, start, stop = calling_fn.source:match('@([^[]*)%[([0-9]+):([0-9]+)]')
           assert(filename)
-          local file = read_file(filename):sub(tonumber(start), tonumber(stop))
-          local err_line = get_line(file, calling_fn.currentline):sub(1, -2)
+          local file = files.read(filename)
+          local err_line = files.get_line(file, calling_fn.currentline)
           local offending_statement = colored.bright(colored.red(err_line:match("^[ ]*(.*)")))
           if calling_fn.name then
             do
@@ -119,7 +129,7 @@ print_error = function(error_message, stack_offset)
         else
           local file
           ok, file = pcall(function()
-            return read_file(calling_fn.short_src)
+            return files.read(calling_fn.short_src)
           end)
           if not ok then
             file = nil
@@ -128,11 +138,12 @@ print_error = function(error_message, stack_offset)
           if name == nil then
             local search_level = level
             local _info = debug.getinfo(search_level)
-            while _info and (_info.func == pcall or _info.func == xpcall) do
+            while true do
               search_level = search_level + 1
               _info = debug.getinfo(search_level)
-            end
-            if _info then
+              if not (_info) then
+                break
+              end
               for i = 1, 999 do
                 local varname, val = debug.getlocal(search_level, i)
                 if not varname then
@@ -177,16 +188,19 @@ print_error = function(error_message, stack_offset)
             end
           end
           if file then
-            local err_line = get_line(file, line_num):sub(1, -2)
+            local err_line = files.get_line(file, line_num)
             local offending_statement = colored.bright(colored.red(err_line:match("^[ ]*(.*)$")))
             line = line .. ("\n        " .. offending_statement)
           end
         end
       end
-      io.stderr:write("    " .. tostring(line) .. "\n")
+      io.stderr:write(line, "\n")
       if calling_fn.istailcall then
         io.stderr:write("    " .. tostring(colored.dim(colored.white("  (...tail calls...)"))) .. "\n")
       end
+      if calling_fn.func == stop_fn then
+        break
+      end
       _continue_0 = true
     until true
     if not _continue_0 then
@@ -195,18 +209,17 @@ print_error = function(error_message, stack_offset)
   end
   return io.stderr:flush()
 end
-local error_handler
-error_handler = function(error_message)
-  print_error(error_message)
-  local EXIT_FAILURE = 1
-  return os.exit(EXIT_FAILURE)
-end
-local run_safely
-run_safely = function(fn)
+local guard
+guard = function(fn)
+  local error_handler
+  error_handler = function(error_message)
+    print_error(error_message, error_handler, fn)
+    local EXIT_FAILURE = 1
+    return os.exit(EXIT_FAILURE)
+  end
   return xpcall(fn, error_handler)
 end
 return {
-  run_safely = run_safely,
-  print_error = print_error,
-  error_handler = error_handler
+  guard = guard,
+  print_error = print_error
 }
diff --git a/error_handling.moon b/error_handling.moon
index 5341348..f28a8c8 100644
--- a/error_handling.moon
+++ b/error_handling.moon
@@ -31,24 +31,28 @@ debug.getinfo = (thread,f,what)->
             if info.lastlinedefined
                 info.lastlinedefined = assert(map[info.lastlinedefined])
             info.short_src = info.source\match('@([^[]*)') or info.short_src
+            -- TODO: get name properly
+            info.name = if info.name
+                if tmp = info.name\match("^A_([a-zA-Z0-9_]*)$")
+                    tmp\gsub("_"," ")\gsub("x([0-9A-F][0-9A-F])", => string.char(tonumber(@, 16)))
+                else info.name
+            else "main chunk"
     return info
 
-print_error = (error_message, stack_offset=3)->
+print_error = (error_message, start_fn, stop_fn)->
     io.stderr\write("#{colored.red "ERROR:"} #{colored.bright colored.red (error_message or "")}\n")
     io.stderr\write("stack traceback:\n")
 
-    get_line = (file, line_no)->
-        start = LINE_STARTS[file][line_no] or 1
-        stop = (LINE_STARTS[file][line_no+1] or 0) - 1
-        return file\sub(start, stop)
-
-    level = stack_offset
+    level = 1
+    found_start = false
     while true
         -- TODO: reduce duplicate code
         calling_fn = debug_getinfo(level)
         if not calling_fn then break
-        if calling_fn.func == run then break
         level += 1
+        unless found_start
+            if calling_fn.func == start_fn then found_start = true
+            continue
         name = calling_fn.name and "function '#{calling_fn.name}'" or nil
         if calling_fn.linedefined == 0 then name = "main chunk"
         if name == "run_lua_fn" then continue
@@ -63,8 +67,8 @@ print_error = (error_message, stack_offset=3)->
             --calling_fn.short_src = calling_fn.source\match('"([^[]*)')
             filename,start,stop = calling_fn.source\match('@([^[]*)%[([0-9]+):([0-9]+)]')
             assert(filename)
-            file = read_file(filename)\sub(tonumber(start),tonumber(stop))
-            err_line = get_line(file, calling_fn.currentline)\sub(1,-2)
+            file = files.read(filename)
+            err_line = files.get_line(file, calling_fn.currentline)
             offending_statement = colored.bright(colored.red(err_line\match("^[ ]*(.*)")))
             -- TODO: get name properly
             name = if calling_fn.name
@@ -74,16 +78,16 @@ print_error = (error_message, stack_offset=3)->
             else "main chunk"
             line = colored.yellow("#{filename}:#{calling_fn.currentline} in #{name}\n        #{offending_statement}")
         else
-            ok, file = pcall ->read_file(calling_fn.short_src)
+            ok, file = pcall ->files.read(calling_fn.short_src)
             if not ok then file = nil
             local line_num
             if name == nil
                 search_level = level
                 _info = debug.getinfo(search_level)
-                while _info and (_info.func == pcall or _info.func == xpcall)
+                while true
                     search_level += 1
                     _info = debug.getinfo(search_level)
-                if _info
+                    break unless _info
                     for i=1,999
                         varname, val = debug.getlocal(search_level, i)
                         if not varname then break
@@ -113,21 +117,21 @@ print_error = (error_message, stack_offset=3)->
                     line = colored.blue("#{calling_fn.short_src}:#{calling_fn.currentline} in #{name or '?'}")
 
             if file
-                err_line = get_line(file, line_num)\sub(1,-2)
+                err_line = files.get_line(file, line_num)
                 offending_statement = colored.bright(colored.red(err_line\match("^[ ]*(.*)$")))
                 line ..= "\n        "..offending_statement
-        io.stderr\write("    #{line}\n")
+        io.stderr\write(line,"\n")
         if calling_fn.istailcall
             io.stderr\write("    #{colored.dim colored.white "  (...tail calls...)"}\n")
+        if calling_fn.func == stop_fn then break
 
     io.stderr\flush!
 
-error_handler = (error_message)->
-    print_error error_message
-    EXIT_FAILURE = 1
-    os.exit(EXIT_FAILURE)
-
-run_safely = (fn)->
+guard = (fn)->
+    error_handler = (error_message)->
+        print_error error_message, error_handler, fn
+        EXIT_FAILURE = 1
+        os.exit(EXIT_FAILURE)
     xpcall(fn, error_handler)
 
-return {:run_safely, :print_error, :error_handler}
+return {:guard, :print_error}
diff --git a/nomsu.lua b/nomsu.lua
index 3bc8fb8..939dda1 100644
--- a/nomsu.lua
+++ b/nomsu.lua
@@ -53,13 +53,14 @@ end
 local EXIT_SUCCESS, EXIT_FAILURE = 0, 1
 local usage = [=[Nomsu Compiler
 
-Usage: (nomsu | lua nomsu.lua | moon nomsu.moon) [-V version] [-O] [-v] [-c] [-s] [-I file] [--help | -h] [--version] [file [nomsu args...]]
+Usage: (nomsu | lua nomsu.lua | moon nomsu.moon) [-V version] [-O] [-v] [-c] [-s] [-t] [-I file] [--help | -h] [--version] [file [nomsu args...]]
 
 OPTIONS
     -O Run the compiler in optimized mode (use precompiled .lua versions of Nomsu files, when available).
     -v Verbose: print compiled lua code.
     -c Compile the input files into a .lua files.
     -s Check the input files for syntax errors.
+    -t Run tests.
     -I <file> Add an additional input file or directory.
     -h/--help Print this message.
     --version Print the version number and exit.
@@ -93,9 +94,11 @@ local parser = re.compile([[    args <- {| (flag ";")* (({~ file ~} -> add_file)
       / ("-I" (";")? ({~ file ~} -> add_file))
       / ({:check_syntax: ("-s" -> true):})
       / ({:compile: ("-c" -> true):})
+      / {:run_tests: ("-t" -> true) :}
       / {:verbose: ("-v" -> true) :}
       / {:help: (("-h" / "--help") -> true) :}
       / {:version: ("--version" -> true) :}
+      / {:debugger: ("-d" (";")? {([^;])*}) :}
       / {:requested_version: "-V" ((";")? {([0-9.])+})? :}
     file <- ("-" -> "stdin") / {[^;]+}
 ]], {
@@ -153,6 +156,13 @@ run = function()
     end
     return true
   end
+  local tests = { }
+  if args.run_tests then
+    nomsu.COMPILE_ACTIONS["test %"] = function(self, tree, _body)
+      table.insert(tests, _body)
+      return LuaCode("")
+    end
+  end
   local get_file_and_source
   get_file_and_source = function(filename)
     local file, source
@@ -188,6 +198,7 @@ run = function()
           tree
         }
       end
+      tests = { }
       for _index_0 = 1, #tree do
         local chunk = tree[_index_0]
         local lua = nomsu:compile(chunk):as_statements("return ")
@@ -198,6 +209,16 @@ run = function()
         end
         nomsu:run_lua(lua)
       end
+      if args.run_tests and #tests > 0 then
+        for _index_0 = 1, #tests do
+          local t = tests[_index_0]
+          local lua = nomsu:compile(t)
+          if lua_handler then
+            lua_handler(tostring(lua))
+          end
+          nomsu:run_lua(lua)
+        end
+      end
     end
   end
   local parse_errs = { }
@@ -294,9 +315,11 @@ say ".."
     end
   end
 end
-local has_ldt, ldt = pcall(require, 'ldt')
-if has_ldt then
-  return ldt.guard(run)
+local debugger = require(args.debugger or 'error_handling')
+local guard
+if type(debugger) == 'function' then
+  guard = debugger
 else
-  return Errhand.run_safely(run)
+  guard = debugger.guard or debugger.call or debugger.wrap or debugger.run
 end
+return guard(run)
diff --git a/nomsu.moon b/nomsu.moon
index 09ee2bd..0bb4752 100755
--- a/nomsu.moon
+++ b/nomsu.moon
@@ -11,13 +11,14 @@ EXIT_SUCCESS, EXIT_FAILURE = 0, 1
 usage = [=[
 Nomsu Compiler
 
-Usage: (nomsu | lua nomsu.lua | moon nomsu.moon) [-V version] [-O] [-v] [-c] [-s] [-I file] [--help | -h] [--version] [file [nomsu args...]]
+Usage: (nomsu | lua nomsu.lua | moon nomsu.moon) [-V version] [-O] [-v] [-c] [-s] [-t] [-I file] [--help | -h] [--version] [file [nomsu args...]]
 
 OPTIONS
     -O Run the compiler in optimized mode (use precompiled .lua versions of Nomsu files, when available).
     -v Verbose: print compiled lua code.
     -c Compile the input files into a .lua files.
     -s Check the input files for syntax errors.
+    -t Run tests.
     -I <file> Add an additional input file or directory.
     -h/--help Print this message.
     --version Print the version number and exit.
@@ -49,9 +50,11 @@ parser = re.compile([[
       / ("-I" (";")? ({~ file ~} -> add_file))
       / ({:check_syntax: ("-s" -> true):})
       / ({:compile: ("-c" -> true):})
+      / {:run_tests: ("-t" -> true) :}
       / {:verbose: ("-v" -> true) :}
       / {:help: (("-h" / "--help") -> true) :}
       / {:version: ("--version" -> true) :}
+      / {:debugger: ("-d" (";")? {([^;])*}) :}
       / {:requested_version: "-V" ((";")? {([0-9.])+})? :}
     file <- ("-" -> "stdin") / {[^;]+}
 ]], {
@@ -99,6 +102,12 @@ run = ->
         return false if args.compile and input_files[f]
         return true
 
+    tests = {}
+    if args.run_tests
+        nomsu.COMPILE_ACTIONS["test %"] = (tree, _body)=>
+            table.insert tests, _body
+            return LuaCode ""
+
     get_file_and_source = (filename)->
         local file, source
         if filename == 'stdin'
@@ -124,12 +133,18 @@ run = ->
             -- Each chunk's compilation is affected by the code in the previous chunks
             -- (typically), so each chunk needs to compile and run before the next one
             -- compiles.
+            tests = {}
             for chunk in *tree
                 lua = nomsu\compile(chunk)\as_statements("return ")
                 lua\declare_locals!
                 lua\prepend "-- File: #{source.filename\gsub("\n.*", "...")}\n"
                 if lua_handler then lua_handler(tostring(lua))
                 nomsu\run_lua(lua)
+            if args.run_tests and #tests > 0
+                for t in *tests
+                    lua = nomsu\compile(t)
+                    if lua_handler then lua_handler(tostring(lua))
+                    nomsu\run_lua(lua)
 
     parse_errs = {}
     for f in *file_queue
@@ -201,8 +216,7 @@ say ".."
             elseif not ok
                 Errhand.print_error ret
 
-has_ldt, ldt = pcall(require,'ldt')
-if has_ldt
-    ldt.guard(run)
-else
-    Errhand.run_safely(run)
+debugger = require(args.debugger or 'error_handling')
+guard = if type(debugger) == 'function' then debugger
+else debugger.guard or debugger.call or debugger.wrap or debugger.run
+guard(run)
diff --git a/nomsu_compiler.lua b/nomsu_compiler.lua
index 8c01860..b2c034a 100644
--- a/nomsu_compiler.lua
+++ b/nomsu_compiler.lua
@@ -301,13 +301,13 @@ do
       return lua
     end,
     ["Lua %"] = function(self, tree, _code)
-      local lua = LuaCode.Value(_code.source, "LuaCode(", repr(tostring(_code.source)))
+      local lua = LuaCode.Value(tree.source, "LuaCode(", repr(tostring(_code.source)))
       add_lua_string_bits(self, lua, _code)
       lua:append(")")
       return lua
     end,
     ["Lua value %"] = function(self, tree, _code)
-      local lua = LuaCode.Value(_code.source, "LuaCode.Value(", repr(tostring(_code.source)))
+      local lua = LuaCode.Value(tree.source, "LuaCode.Value(", repr(tostring(_code.source)))
       add_lua_string_bits(self, lua, _code)
       lua:append(")")
       return lua
@@ -332,6 +332,9 @@ do
         end
       end
       return LuaCode(tree.source, "for f in files.walk(", self:compile(_path), ") do nomsu:run_file(f) end")
+    end,
+    ["test %"] = function(self, tree, _body)
+      return LuaCode("")
     end
   }, {
     __index = function(self, stub)
@@ -434,7 +437,7 @@ do
       source = nil
     end
     local lua_string = tostring(lua)
-    local run_lua_fn, err = load(lua_string, nil and tostring(source or lua.source), "t", self)
+    local run_lua_fn, err = load(lua_string, tostring(source or lua.source), "t", self)
     if not run_lua_fn then
       local line_numbered_lua = concat((function()
         local _accum_0 = { }
@@ -447,39 +450,36 @@ do
       end)(), "\n")
       error("Failed to compile generated code:\n" .. tostring(colored.bright(colored.blue(colored.onblack(line_numbered_lua)))) .. "\n\n" .. tostring(err), 0)
     end
-    local source_key = tostring(source or lua.source)
+    source = source or lua.source
+    local source_key = tostring(source)
     if not (SOURCE_MAP[source_key]) then
       local map = { }
-      local offset = 1
-      source = source or lua.source
       local file = files.read(source.filename)
       if not file then
         error("Failed to find file: " .. tostring(source.filename))
       end
       local nomsu_str = tostring(file:sub(source.start, source.stop))
       local lua_line = 1
-      local nomsu_line = files.get_line_number(nomsu_str, source.start)
-      local fn
-      fn = function(s)
+      local nomsu_line = files.get_line_number(file, source.start)
+      local map_sources
+      map_sources = function(s)
         if type(s) == 'string' then
           for nl in s:gmatch("\n") do
             map[lua_line] = map[lua_line] or nomsu_line
             lua_line = lua_line + 1
           end
         else
-          local old_line = nomsu_line
-          if s.source then
-            nomsu_line = files.get_line_number(nomsu_str, s.source.start)
+          if s.source and s.source.filename == source.filename then
+            nomsu_line = files.get_line_number(file, s.source.start)
           end
           local _list_0 = s.bits
           for _index_0 = 1, #_list_0 do
             local b = _list_0[_index_0]
-            fn(b)
+            map_sources(b)
           end
         end
       end
-      fn(lua)
-      map[lua_line] = map[lua_line] or nomsu_line
+      map_sources(lua)
       map[0] = 0
       SOURCE_MAP[source_key] = map
     end
diff --git a/nomsu_compiler.moon b/nomsu_compiler.moon
index 1a7a6e7..83c864c 100644
--- a/nomsu_compiler.moon
+++ b/nomsu_compiler.moon
@@ -189,13 +189,13 @@ with NomsuCompiler
             return lua
 
         ["Lua %"]: (tree, _code)=>
-            lua = LuaCode.Value(_code.source, "LuaCode(", repr(tostring _code.source))
+            lua = LuaCode.Value(tree.source, "LuaCode(", repr(tostring _code.source))
             add_lua_string_bits(@, lua, _code)
             lua\append ")"
             return lua
     
         ["Lua value %"]: (tree, _code)=>
-            lua = LuaCode.Value(_code.source, "LuaCode.Value(", repr(tostring _code.source))
+            lua = LuaCode.Value(tree.source, "LuaCode.Value(", repr(tostring _code.source))
             add_lua_string_bits(@, lua, _code)
             lua\append ")"
             return lua
@@ -216,6 +216,9 @@ with NomsuCompiler
                 for f in files.walk(path)
                     @run_file(f)
             return LuaCode(tree.source, "for f in files.walk(", @compile(_path), ") do nomsu:run_file(f) end")
+
+        ["test %"]: (tree, _body)=>
+            return LuaCode ""
     }, {
         __index: (stub)=>
             if math_expression\match(stub)
@@ -285,35 +288,33 @@ with NomsuCompiler
 
     .run_lua = (lua, source=nil)=>
         lua_string = tostring(lua)
-        run_lua_fn, err = load(lua_string, nil and tostring(source or lua.source), "t", self)
+        run_lua_fn, err = load(lua_string, tostring(source or lua.source), "t", self)
         if not run_lua_fn
             line_numbered_lua = concat(
                 [format("%3d|%s",i,line) for i, line in ipairs files.get_lines(lua_string)],
                 "\n")
             error("Failed to compile generated code:\n#{colored.bright colored.blue colored.onblack line_numbered_lua}\n\n#{err}", 0)
-        source_key = tostring(source or lua.source)
+        source or= lua.source
+        source_key = tostring(source)
         unless SOURCE_MAP[source_key]
             map = {}
-            offset = 1
-            source or= lua.source
             file = files.read(source.filename)
             if not file
                 error "Failed to find file: #{source.filename}"
             nomsu_str = tostring(file\sub(source.start, source.stop))
             lua_line = 1
-            nomsu_line = files.get_line_number(nomsu_str, source.start)
-            fn = (s)->
+            nomsu_line = files.get_line_number(file, source.start)
+            map_sources = (s)->
                 if type(s) == 'string'
                     for nl in s\gmatch("\n")
                         map[lua_line] or= nomsu_line
                         lua_line += 1
                 else
-                    old_line = nomsu_line
-                    if s.source
-                        nomsu_line = files.get_line_number(nomsu_str, s.source.start)
-                    for b in *s.bits do fn(b)
-            fn(lua)
-            map[lua_line] or= nomsu_line
+                    if s.source and s.source.filename == source.filename
+                        nomsu_line = files.get_line_number(file, s.source.start)
+                    for b in *s.bits do map_sources(b)
+            map_sources(lua)
+            --map[lua_line] or= nomsu_line
             map[0] = 0
             -- Mapping from lua line number to nomsu line numbers
             SOURCE_MAP[source_key] = map