function parseAddSubtract(state) {
    var node, operators, name, fn, params;
    node = parseMultiplyDivide(state);
    operators = {
      '+': 'add',
      '-': 'subtract'
    };

    while (operators.hasOwnProperty(state.token)) {
      name = state.token;
      fn = operators[name];
      getTokenSkipNewline(state);
      params = [node, parseMultiplyDivide(state)];
      node = new OperatorNode(name, fn, params);
    }

    return node;
  }
add or subtract
@return {Node} node
@private