code: func (r *Lexer) Null() {
  	if r.token.kind == tokenUndef && r.Ok() {
  		r.FetchToken()
  	}
  	if !r.Ok() || r.token.kind != tokenNull {
  		r.errInvalidToken("null")
  	}
  	r.consume()
  }
docs: // Null verifies that the next token is null and consumes it.