diff --git a/tokenize.l b/tokenize.l
index 0f01fee..fb238ee 100644
--- a/tokenize.l
+++ b/tokenize.l
@@ -274,6 +274,23 @@ eval_add(struct ast *a, struct env *e)
                              NULL));
 }
 
+/* inv <ausdruck::NUM> */
+struct ast *
+eval_inv(struct ast *a, struct env *e)
+{
+  assert(a != NULL && 
+         a->next != NULL);
+  struct ast *op = eval(a->next, e);
+  assert(op != NULL &&
+         op->type == AST_TOK &&
+         op->v.token->type == NUM);
+  return make_ast(AST_TOK, 
+                  make_token(NUM, 
+                             0 - 
+                             op->v.token->v.num,
+                             NULL));
+}
+
 struct ast *
 eval_sym(struct ast *a, struct env *e)
 {
@@ -281,6 +298,8 @@ eval_sym(struct ast *a, struct env *e)
     return eval_def(a, e);
   else if (strcmp(a->v.token->v.str, "lm") == 0) {
   } else if (strcmp(a->v.token->v.str, "if") == 0) {
+  } else if (strcmp(a->v.token->v.str, "inv") == 0) {
+    return eval_inv(a, e);
   } else if (strcmp(a->v.token->v.str, "add") == 0) {
     return eval_add(a, e);
   } else if (strcmp(a->v.token->v.str, "q") == 0) {