code: func lexIdentifier(l *Lexer) lexFunc {
  	str := l.findRegexp(rID)
  	if len(str) == 0 {
  		// this is rotten
  		panic("Identifier expected")
  	}
  
  	l.pos += len(str)
  	l.emit(TokenID)
  
  	return lexExpression
  }
docs: // lexIdentifier scans an ID