code: func StopAt(word string) func(*Parser) {
  	if len(word) > 4 {
  		panic("stop word can't be over four bytes in size")
  	}
  	if strings.ContainsAny(word, " \t\n\r") {
  		panic("stop word can't contain whitespace characters")
  	}
  	return func(p *Parser) { p.stopAt = []byte(word) }
  }
docs: // StopAt configures the lexer to stop at an arbitrary word, treating it
  // as if it were the end of the input. It can contain any characters
  // except whitespace, and cannot be over four bytes in size.
  //
  // This can be useful to embed shell code within another language, as
  // one can use a special word to mark the delimiters between the two.
  //
  // As a word, it will only apply when following whitespace or a
  // separating token. For example, StopAt("$$") will act on the inputs
  // "foo $$" and "foo;$$", but not on "foo '$$'".
  //
  // The match is done by prefix, so the example above will also act on
  // "foo $$bar".