diff --git a/examples/rules.modal b/examples/rules.modal
new file mode 100644
index 0000000..2e01a22
--- /dev/null
+++ b/examples/rules.modal
@@ -0,0 +1,19 @@
+<> rule0 data
+<> rule1 data-1
+<> rule2 
+<> rule3 data-10
+<> rule4 data-100
+
+?(?-) (Undefine an empty rule)
+
+>< rule2
+
+?(?-) (Undefine the last rule)
+
+>< rule4
+
+?(?-) (Undefine a non-existant rule)
+
+>< rule5
+
+a sample program
\ No newline at end of file
diff --git a/makefile b/makefile
index b785838..ccee82f 100644
--- a/makefile
+++ b/makefile
@@ -10,7 +10,7 @@ dest:
 run: all bin/modal
 	@ bin/modal -q examples/hello.modal
 debug: all bin/modal-debug
-	@ bin/modal-debug examples/hello.modal
+	@ bin/modal-debug examples/rules.modal
 test: all bin/modal-debug bin/modal
 	@ bin/modal -v
 	@ bin/modal-debug -q examples/fizzbuzz.modal
diff --git a/src/modal.c b/src/modal.c
index 451279b..0a9ab35 100644
--- a/src/modal.c
+++ b/src/modal.c
@@ -22,6 +22,15 @@ copy(char *src, char *dst, int length)
 	return dst;
 }
 
+static int
+slen(char *s)
+{
+	char *cap = s;
+	while(*cap++)
+		;
+	return cap - s;
+}
+
 static char *
 walk(char *s)
 {
@@ -233,10 +242,10 @@ parse_frag(char **side, char *s)
 {
 	char c, *cap;
 	while((c = *s) && c == ' ') s++;
-	if(c == ')') {
+	if(c == ')' || (c == '<' && s[1] == '>') || (c == '>' && s[1] == '<')) {
 		*side = NULL;
 		return s;
-	} else if((c == '<' && s[1] == '>') || (c == '>' && s[1] == '<') || (c == '(' && s[1] == ')')) {
+	} else if((c == '(' && s[1] == ')')) {
 		*side = NULL;
 		return s + 2;
 	} else {
@@ -257,14 +266,31 @@ find_rule(char *s, char *cap)
 	if(*s == '(') s++, cap--;
 	while(r < rules_) {
 		char *ss = s, *a = r->a;
-		if(a)
-			while(*ss++ == *a++)
-				if(!*a && ss == cap) return r;
+		while(*ss++ == *a++)
+			if(!*a && ss == cap) return r;
 		r++;
 	}
 	return NULL;
 }
 
+static void
+remove_rule(Rule *r)
+{
+	if(r < rules_ - 1) {
+		int distance = slen(r->a) + (r->b != NULL ? slen(r->b) : 0);
+		char *memsrc = (r + 1)->a;
+		copy(memsrc, r->a, dict_ - memsrc);
+		while(r < rules_ - 1) {
+			Rule *next = r + 1;
+			r->id = next->id, r->refs = next->refs;
+			r->a = next->a - distance;
+			r->b = next->b == NULL ? NULL : next->b - distance;
+			r++;
+		}
+	}
+	rules_--;
+}
+
 static int
 rewrite(void)
 {
@@ -272,7 +298,7 @@ rewrite(void)
 	while(*s == ' ') s++;
 	while((c = *s)) {
 		if(c == '(' || spacer(last)) {
-			Rule *r = NULL;
+			Rule *r;
 			/* phase: undefine */
 			if(c == '>' && s[1] == '<') {
 				s += 2;
@@ -280,7 +306,7 @@ rewrite(void)
 				cap = walk(s), r = find_rule(s, cap);
 				if(r != NULL) {
 					if(!quiet) fprintf(stderr, ">< (%s) (%s)\n", r->a ? r->a : "", r->b ? r->b : "");
-					r->a = 0;
+					remove_rule(r);
 				}
 				while(*cap == ' ') cap++;
 				return write_tail(cap);
@@ -310,7 +336,7 @@ rewrite(void)
 			}
 			/* phase: match */
 			for(r = rules; r < rules_; r++)
-				if(r->a != NULL && apply_rule(r, s)) return 1;
+				if(apply_rule(r, s)) return 1;
 		}
 		*dst_++ = last = c, s++;
 	}
@@ -326,7 +352,7 @@ main(int argc, char **argv)
 		return !printf("usage: modal [-vqn] source.modal\n");
 	for(i = 1; i < argc && *argv[i] == '-'; i++) {
 		switch(argv[i][1]) {
-		case 'v': /* version */ return !printf("Modal Interpreter, 17 May 2024.\n");
+		case 'v': /* version */ return !printf("Modal Interpreter, 21 May 2024.\n");
 		case 'q': /* quiet */ quiet = 1; break;
 		case 'p': /* debug */ debug = 1; break;
 		case 'a': /* access */ access = 1; break;