function Cross(params) {
    Transform.call(this, null, params);
  }
Perform a cross-product of a tuple stream with itself.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object):boolean} [params.filter] - An optional filter
  function for selectively including tuples in the cross product.
@param {Array<string>} [params.as] - The names of the output fields.