Use the current op to determine which etuple function to use

The first argument passed to `etuple` determines which of the registered
functions is called. The first argument is generally an
`ExpressionTuple`, the etuplized version of the operator. However, when
using subclasses of `ExpressionTuple` to customize evaluation, we want
`etuple` to depend on the current operator and not its etuplized
version.

etuples/dispatch.py

@@ -101,6 +101,11 @@ def apply_ExpressionTuple(rator, rands):
 operator, arguments, term = rator, rands, apply
 
 
+@dispatch(object)
+def etuplize_fn(op):
+    return etuple
+
+
 @dispatch(object)
 def etuplize(
     x,
@@ -140,6 +145,7 @@ def etuplize_step(
         return_bad_args=return_bad_args,
         convert_ConsPairs=convert_ConsPairs,
     ):
+
         if isinstance(x, ExpressionTuple):
             yield x
             return
@@ -182,6 +188,6 @@ def etuplize_step(
                 )
                 et_args.append(e)
 
-        yield etuple(et_op, *et_args, evaled_obj=x)
+        yield etuplize_fn(op)(et_op, *et_args, evaled_obj=x)
 
     return trampoline_eval(etuplize_step(x))