parse.y:4867
static void
rb_backref_error(node)
    NODE *node;
{
    switch (nd_type(node)) {
      case NODE_NTH_REF:
	rb_compile_error("Can't set variable $%d", node->nd_nth);
	break;
      case NODE_BACK_REF:
	rb_compile_error("Can't set variable $%c", (int)node->nd_nth);
	break;
    }
}
