comment about number of steps

theano/scan_module/scan_op.py

@@ -1253,6 +1253,12 @@ class Scan(PureOp):
         outs = self(*inputs)
         if not isinstance(outs, (list, tuple)):
             outs = [outs]
+        # `grad_step` equals the number of steps the original scan node has
+        # done (if the original scan is a while loop than this number is the
+        # length of the output sequence)
+        # We do not know what kind of outputs the original scan has, so we
+        # try first to see if it has a nit_sot output, then a sit_sot and
+        # then a mit_sot
         if self.n_nit_sot > 0:
             grad_steps = self.outer_nitsot_outs(outs)[0].shape[0]
         elif self.n_sit_sot > 0: