anchor.diff:  
   /**
    * The Parser creates a mathematical [Expression] from a given input string.
  - * 
  + *
    * It uses a [Lexer] to create a RPN token stream and then builds the
    * expression.
  - * 
  + *
    * Usage example:
    *     Parser p = new Parser();
    *     Expression exp = p.parse("(x^2 + cos(y)) / 3");
  
positive.code:       Expression currExpr, left, right;
  
        switch(currToken.type) {
          case TokenType.VAL: 
            currExpr = new Number(double.parse(currToken.text));
            break;
          case TokenType.VAR:
  
negative.code: 
  /**
   * A token type. Access token types via the static fields.
   *
   * For example, to access the token type PLUS:
   *     plusType = TokenType.PLUS;
   *
   * The type defines the `priority` (precedence) of the token.
   *     (+,-) < (*,/) < (^) < functions < (-u)
   *
   * It also defines the associativity of the token. True stands for
   * left-associative, false for right-associative.
   */
  