func (in *Input) ifdef(truth bool) {
	name := in.macroName()
	in.expectNewline("#if[n]def")
	if !in.enabled() {
		truth = false
	} else if _, defined := in.macros[name]; !defined {
		truth = !truth
	}
	in.ifdefStack = append(in.ifdefStack, truth)
}

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