started work on fixing gradient issues

theano/gradient.py

@@ -36,7 +36,14 @@ def grad_sources_inputs(sources, graph_inputs, warn_type=True):
     them)
 
     :rtype: dictionary whose keys and values are of type `Variable`
-    :return: mapping from each Variable encountered in the backward traversal to its gradient.
+    :return: mapping from each Variable encountered in the backward traversal to the gradient with respect to that Variable.
+
+    It is assumed that there is some objective J shared between all members of
+    sources, so that for each v, gradient-on-v is the gradient of J with respect to v
+
+
+
+
     """
     gmap = {}
     for (r, g_r) in sources:
@@ -125,3 +132,14 @@ def unimplemented_grad(op, x_pos, x):
     """
     msg = '%s.grad not implemented for input %i'%(op, x_pos)
     return Raise(msg=msg)(x)
+
+
+class GradientUndefined(Exception): pass
+
+def undefined_grad(op, x_pos, x):
+    msg = "Undefined gradient - do not use in computations"
+    exc = RuntimeError
+    return Raise(msg=msg, exc=exc)(x)
+
+def grad(self, inputs, out_storage):
+    return [g_x0, undefined_grad(self, 1, inputs[1])]