code: func (p *parser) consumeParenthesis() bool {
  	if p.i < len(p.s) && p.s[p.i] == '(' {
  		p.i++
  		p.skipWhitespace()
  		return true
  	}
  	return false
  }
docs: // consumeParenthesis consumes an opening parenthesis and any following
  // whitespace. It returns true if there was actually a parenthesis to skip.