// The opening parenthesis has been absorbed.
func (in *Input) argsFor(macro *Macro) map[string][]Token {
	var args [][]Token
	// One macro argument per iteration. Collect them all and check counts afterwards.
	for argNum := 0; ; argNum++ {
		tokens, tok := in.collectArgument(macro)
		args = append(args, tokens)
		if tok == ')' {
			break
		}
	}
	// Zero-argument macros are tricky.