if len(in.ifdefStack) == 0 {
		in.Error("unmatched #else")
	}
	if len(in.ifdefStack) == 1 || in.ifdefStack[len(in.ifdefStack)-2] {
		in.ifdefStack[len(in.ifdefStack)-1] = !in.ifdefStack[len(in.ifdefStack)-1]
	}
}

// #endif processing.
func (in *Input) endif() {
	in.expectNewline("#endif")
	if len(in.ifdefStack) == 0 {
		in.Error("unmatched #endif")
	}