diff --git a/Makefile b/Makefile
index e1ef268..5e50c9c 100644
--- a/Makefile
+++ b/Makefile
@@ -7,7 +7,7 @@ all: bpeg
 clean:
 	rm -f bpeg
 
-bpeg: bpeg.c bpeg.h vm.h utils.h
+bpeg: bpeg.c bpeg.h utils.h
 	cc $(CFLAGS) $(OFLAGS) $< -o $@
 
 .PHONY: all clean
diff --git a/bpeg.c b/bpeg.c
index d09dba3..31c2016 100644
--- a/bpeg.c
+++ b/bpeg.c
@@ -219,7 +219,7 @@ static match_t *match(const char *str, vm_op_t *op)
                 m->end = m->start;
             }
             // TODO: handle captures
-            m->replacement = op->args.replace.replacement;
+            m->capture.replacement = op->args.replace.replacement;
             return m;
         }
         case VM_REF: {
@@ -666,16 +666,19 @@ static vm_op_t *compile_bpeg(const char *str)
     return op;
 }
 
-static void load_def(const char *name, const char *def)
+static vm_op_t *load_def(const char *name, const char *def)
 {
     defs[ndefs].name = name;
-    defs[ndefs].op = compile_bpeg(def);
+    vm_op_t *op = compile_bpeg(def);
+    defs[ndefs].op = op;
     ++ndefs;
+    return op;
 }
 
 static void load_defs(void)
 {
-    load_def("_", "` /\\t/\\n/\\r");
+    load_def("_", "*(` /\\t/\\n/\\r)");
+    load_def("__", "+(` /\\t/\\n/\\r)");
     load_def("nl", "\\n");
     load_def("crlf", "\\r\\n");
     load_def("abc", "`a,z");
@@ -696,15 +699,28 @@ static void load_defs(void)
 
 int main(int argc, char *argv[])
 {
+    check(argc == 3, "Usage: bpeg <pat> <str>");
     load_defs();
 
-    char *lang = argc > 1 ? argv[1] : "'x''y'";
+    const char *lang = argc > 1 ? argv[1] : "'x''y'";
     vm_op_t *op = compile_bpeg(lang);
     check(op, "Failed to compile_bpeg input");
     op = expand_choices(op);
     
-    // TODO: check for semicolon and more rules
-
+    const char *defs = after_spaces(op->end);
+    while (*defs == ';') {
+      defs = after_spaces(++defs);
+      const char *name = defs;
+      check(isalpha(*name), "Definition must begin with a name");
+      while (isalpha(*defs)) ++defs;
+      name = strndup(name, (size_t)(defs-name));
+      defs = after_spaces(defs);
+      check(*defs == '=', "Expected '=' in definition");
+      ++defs;
+      vm_op_t *def = load_def(name, defs);
+      check(def, "Couldn't load definition");
+      defs = def->end;
+    }
 
     char *str = argc > 2 ? argv[2] : "xyz";
 
@@ -725,3 +741,5 @@ int main(int argc, char *argv[])
 
     return 0;
 }
+
+//vim: ts=4
diff --git a/bpeg.h b/bpeg.h
index 8bc813a..a855e8a 100644
--- a/bpeg.h
+++ b/bpeg.h
@@ -8,18 +8,75 @@
 #include <unistd.h>
 
 #include "utils.h"
-#include "vm.h"
 
+/*
+ * Pattern matching result object
+ */
 typedef struct match_s {
+    // Where the match starts and ends (end is after the last character)
     const char *start, *end;
     union {
         unsigned int is_capture:1;
         const char *name;
+        const char *replacement;
     } capture;
-    const char *replacement;
     struct match_s *child, *nextsibling;
 } match_t;
 
+/*
+ * BPEG virtual machine opcodes
+ */
+enum VMOpcode {
+    VM_EMPTY = 0,
+    VM_ANYCHAR = 1,
+    VM_STRING,
+    VM_RANGE,
+    VM_NOT,
+    VM_UPTO,
+    VM_UPTO_AND,
+    VM_REPEAT,
+    VM_BEFORE,
+    VM_AFTER,
+    VM_CAPTURE,
+    VM_OTHERWISE,
+    VM_CHAIN,
+    VM_REPLACE,
+    VM_REF,
+};
+
+/*
+ * A struct reperesenting a BPEG virtual machine operation
+ */
+typedef struct vm_op_s {
+    enum VMOpcode op;
+    const char *start, *end;
+    // Length of the match, if constant, otherwise -1
+    ssize_t len;
+    union {
+        const char *s;
+        struct {
+            char low, high;
+        } range;
+        struct {
+            ssize_t min, max;
+            struct vm_op_s *sep, *repeat_pat;
+        } repetitions;
+        struct {
+            struct vm_op_s *first, *second;
+        } multiple;
+        struct {
+            struct vm_op_s *replace_pat;
+            const char *replacement;
+        } replace;
+        struct {
+            struct vm_op_s *capture_pat;
+            char *name;
+        } capture;
+        struct vm_op_s *pat;
+    } args;
+} vm_op_t;
+
+
 static match_t *free_match(match_t *m);
 static match_t *match(const char *str, vm_op_t *op);
 static void set_range(vm_op_t *op, ssize_t min, ssize_t max, vm_op_t *pat, vm_op_t *sep);
diff --git a/vm.h b/vm.h
deleted file mode 100644
index 2123c35..0000000
--- a/vm.h
+++ /dev/null
@@ -1,50 +0,0 @@
-/*
- * vm.h - Source code for the BPEG virtual machine datatypes
- */
-
-enum VMOpcode {
-    VM_EMPTY = 0,
-    VM_ANYCHAR = 1,
-    VM_STRING,
-    VM_RANGE,
-    VM_NOT,
-    VM_UPTO,
-    VM_UPTO_AND,
-    VM_REPEAT,
-    VM_BEFORE,
-    VM_AFTER,
-    VM_CAPTURE,
-    VM_OTHERWISE,
-    VM_CHAIN,
-    VM_REPLACE,
-    VM_REF,
-};
-
-typedef struct vm_op_s {
-    enum VMOpcode op;
-    const char *start, *end;
-    ssize_t len;
-    union {
-        const char *s;
-        struct {
-            char low, high;
-        } range;
-        struct {
-            ssize_t min, max;
-            struct vm_op_s *sep, *repeat_pat;
-        } repetitions;
-        struct {
-            struct vm_op_s *first, *second;
-        } multiple;
-        struct {
-            struct vm_op_s *replace_pat;
-            const char *replacement;
-        } replace;
-        struct {
-            struct vm_op_s *capture_pat;
-            char *name;
-        } capture;
-        struct vm_op_s *pat;
-    } args;
-} vm_op_t;
-