added comments

theano/scalar/basic.py

@@ -64,6 +64,16 @@ def constant(x):
 
 class Scalar(Type):
 
+    """
+        Internal class, should not be used by clients
+        Primarily used by tensor.elemwise and tensor.reduce
+        Analogous to TensorType, but for zero-dimensional objects
+        Maps directly to C primitives
+
+        TODO: refactor to be named ScalarType for consistency with TensorType
+    """
+
+
     def __init__(self, dtype):
         if dtype == 'floatX':
             dtype = config.floatX

theano/tensor/elemwise.py

@@ -537,7 +537,10 @@ class Elemwise(Op):
 
     def grad(self, inputs, ograds):
         # Gradients (especially on the final costs) don't have to be symbolic
+        # e.g., ograds will be [ 1. ] if your objective is c and the output
+        # of the current apply node is c
         ograds = map(as_tensor_variable, ograds)
+
         scalar_inputs = [Scalar(dtype = t.type.dtype)() for t in inputs]
         scalar_ograds = [Scalar(dtype = ograd.type.dtype)() for ograd in ograds]
         scalar_igrads = self.scalar_op.grad(scalar_inputs, scalar_ograds)