Fix a theano.pp printing error and add a test for it.

theano/tensor/subtensor.py

@@ -967,7 +967,7 @@ class SubtensorPrinter:
         elif isinstance(r.owner.op, Subtensor):
             idxs = r.owner.op.idx_list
             inputs = list(r.owner.inputs)
-            input = inputs.pop()
+            input = inputs.pop(0)
             sidxs = []
             old_precedence = getattr(pstate, 'precedence', None)
             try:

theano/tests/test_printing.py

@@ -738,3 +738,9 @@ def test_printing_scan():
                         allow_input_downcast=True)
     theano.printing.pydotprint(output, scan_graphs=True)
     theano.printing.pydotprint(f, scan_graphs=True)
+
+
+def test_subtensor():
+    x = theano.tensor.dvector()
+    y = x[1]
+    assert theano.pp(y) == "<TensorType(float64, vector)>[Constant{1}]"