frontend/define.c, ntharg(), #4/15 express some facts

src/frontend/define.c

@@ -407,8 +407,10 @@ trcopy(struct pnode *tree, char *args, struct pnode *nn)
 static struct pnode *
 ntharg(int num, struct pnode *args)
 {
+    // fact: num >= 1 for all known invocations of ntharg()
         while (--num > 0) {
             if (args && args->pn_op && (args->pn_op->op_num != PT_OP_COMMA)) {
+                // fact: num >= 1 because of `while' condition
                 if (num == 1)
                     break;
                 return NULL;