code: func lexIdentifier(l *lexer) stateFn {
  	for isAlphaNumeric(l.next()) {
  		// absorb
  	}
  	l.backup()
  	l.emit(ItemIdentifier)
  	return lexStatements
  }
docs: // lexIdentifier scans an alphanumeric identifier. The next character
  // is known to be a letter.