def cond(pred, true_fn, false_fn, name: nil)
      _op(:case, [pred], false_fn, true_fn, name: name)
    end
def cond(pred, true_fn, false_fn, name: nil)
      _op(:case, [pred], false_fn, true_fn, name: name)
    end
Return true_fn() if the predicate pred is true else false_fn().