def where(condition, true_t = nil, false_t = nil, name: nil)
      _op(:where, condition, true_t, false_t, name: name)
    end
def where(condition, true_t = nil, false_t = nil, name: nil)
      _op(:where, condition, true_t, false_t, name: name)
    end
Return the elements, either from x or y, depending on the condition.