code: function binaryOperator(queryObject, actualField, key) {
      return actualField + OPS[key] + stringify(queryObject, actualField);
  }
docs: Converts binary operator queries into string by applying operator and
  recursively invokes parsing process for rvalue.
  
  @param {object} queryObject Which contains the operator.
  @param {string} actualField Current field of the object which the filter is operating on.
  @param {string} key The current operation key.
  @returns {string}