diff --git a/b2/SUBST b/b2/SUBST
index 039ada9..595a855 100644
--- a/b2/SUBST
+++ b/b2/SUBST
@@ -1,10 +1,12 @@
+FN=DNP { ignore }
+
 REF=R[0-9]* {
 	T=R
 	VAL=(#R) { R=$1 }
 	TOL <= 5%
 	FN=*% { TOL<=$$ }
 	break REF
-//	end break again
+//	end break again ignore
 }
 
 /*
diff --git a/b2/boom.c b/b2/boom.c
index 83ea06e..55d7a2e 100644
--- a/b2/boom.c
+++ b/b2/boom.c
@@ -48,7 +48,10 @@ static void do_substitutions(void)
 	const struct param *var;
 	const struct part **parts, **p;
 
-	out = substitute(substitutions, vars);
+	if (!substitute(substitutions, vars, &out)) {
+		fprintf(stderr, "ignore\n");
+		return;
+	}
 	if (select_parts) {
 		parts = select_parametric(out, &hierarchy);
 		if (!parts) {
diff --git a/b2/lang.y b/b2/lang.y
index 9d7d022..64d5ee4 100644
--- a/b2/lang.y
+++ b/b2/lang.y
@@ -613,9 +613,12 @@ block:
 		}
 	| WORD
 		{
-			if (strcmp($1, "end"))
+			if (!strcmp($1, "end"))
+				$$ = subst_end();
+			else if (!strcmp($1, "ignore"))
+				$$ = subst_ignore();
+			else
 				yyerrorf("unknown keyword \"%s\"", $1);
-			$$ = subst_end();
 		}
 	;
 
diff --git a/b2/subex.c b/b2/subex.c
index 97b8342..fe058d0 100644
--- a/b2/subex.c
+++ b/b2/subex.c
@@ -30,9 +30,9 @@
 
 static const char *fn = NULL, *f[FIELDS];
 
-/* Jump target that can never be reached. */
+/* Jump targets that can never be reached. */
 static struct subst jump_end;
-
+static struct subst jump_ignore;
 
 
 static char *canonicalize(const char *s, char unit)
@@ -206,6 +206,8 @@ static const struct subst *recurse_sub(const struct subst *sub,
 			break;
 		case st_end:
 			return &jump_end;
+		case st_ignore:
+			return &jump_ignore;
 		case st_break:
 			sub = sub->u.jump->next;
 			continue;
@@ -221,9 +223,9 @@ static const struct subst *recurse_sub(const struct subst *sub,
 }
 
 
-struct param *substitute(const struct subst *sub, const struct param *in)
+int substitute(const struct subst *sub, const struct param *in,
+    struct param **out)
 {
-	struct param *out = NULL;
 	int i;
 	char tmp[4];
 
@@ -234,6 +236,6 @@ struct param *substitute(const struct subst *sub, const struct param *in)
 			f[i] = unique(tmp);
 		}
 	}
-	recurse_sub(sub, in, NULL, NULL, NULL, &out);
-	return out;
+	*out = NULL;
+	return recurse_sub(sub, in, NULL, NULL, NULL, out) != &jump_ignore;
 }
diff --git a/b2/subex.h b/b2/subex.h
index e280bdc..a664ae0 100644
--- a/b2/subex.h
+++ b/b2/subex.h
@@ -17,6 +17,7 @@
 #include "subst.h"
 
 
-struct param *substitute(const struct subst *sub, const struct param *in);
+int substitute(const struct subst *sub, const struct param *in,
+    struct param **out);
 
 #endif /* !SUBEX_H */
diff --git a/b2/subst.c b/b2/subst.c
index 6cf2058..e0c3718 100644
--- a/b2/subst.c
+++ b/b2/subst.c
@@ -245,6 +245,12 @@ struct subst *subst_end(void)
 }
 
 
+struct subst *subst_ignore(void)
+{
+	return alloc_subst(st_ignore);
+}
+
+
 struct subst *subst_break(const char *block)
 {
 	struct subst *sub;
@@ -357,6 +363,8 @@ static void recurse_fin(struct subst *sub, const struct parent *parent)
 			break;
 		case st_end:
 			break;
+		case st_ignore:
+			break;
 		case st_break:
 		case st_again:
 			sub->u.jump = resolve_jump(sub->u.tmp, parent);
@@ -425,6 +433,9 @@ static void recurse_dump(FILE *file, const struct subst *sub, int level)
 		case st_end:
 			fprintf(file, "end\n");
 			break;
+		case st_ignore:
+			fprintf(file, "ignore\n");
+			break;
 		case st_break:
 			fprintf(file, "break %s\n", sub->u.jump->u.match.src);
 			break;
diff --git a/b2/subst.h b/b2/subst.h
index bb68445..22d872c 100644
--- a/b2/subst.h
+++ b/b2/subst.h
@@ -37,11 +37,12 @@ struct chunk {
 };
 
 enum subst_type {
-	st_match,
-	st_assign,
-	st_end,
-	st_break,
-	st_again,
+	st_match,	/* try to match a variable */
+	st_assign,	/* assign to a variable */
+	st_end,		/* end the substitutions, accepting the part */
+	st_ignore,	/* ignore the part */
+	st_break,	/* jump to an outer block */
+	st_again,	/* repeat an outer block */
 };
 
 struct subst {
@@ -73,6 +74,7 @@ struct subst {
 struct subst *subst_match(const char *src, const char *re);
 struct subst *subst_assign(const char *dst, enum relop op, const char *pat);
 struct subst *subst_end(void);
+struct subst *subst_ignore(void);
 struct subst *subst_break(const char *block);
 struct subst *subst_again(const char *block);