code: func (s *Scanner) emitPrefixOrChar(t tokenType, prefix string) *Token {
  	if strings.HasPrefix(s.input[s.pos:], prefix) {
  		return s.emitSimple(t, prefix)
  	}
  	return s.emitSimple(TokenChar, string(prefix[0]))
  }
docs: // emitPrefixOrChar returns a Token for type t if the current position
  // matches the given prefix. Otherwise it returns a Char token using the
  // first character from the prefix.
  //
  // The prefix is known to have only ASCII characters and to not have a newline.