Added some documentation and a minor optimization.

theano/tensor/opt.py

@@ -1550,16 +1550,23 @@ def local_useless_elemwise(node):
     mul(x) -> x
     add(x) -> x
     identity(x) -> x
+    and(x,1) -> x
+    and(x,0) -> zeros_like(x)
+    or(x,0) -> x
+    or(x,1) -> ones_like(x)
+    xor(x,x) -> zeros_like(x)
+    le(x,x) -> ones_like(x)
+    ge(x,x) -> ones_like(x)
 
     """
     if isinstance(node.op, T.Elemwise):
         def zeros_like(node, in_idx):
-            #it is the same var in the graph. That will always be true
+            # it is the same var in the graph. That will always be true
             return [T.fill(node.inputs[in_idx],
                            T.constant(0.0, dtype=node.outputs[0].type.dtype))]
-            
+
         def ones_like(node, in_idx):
-            #it is the same var in the graph. That will always be true
+            # it is the same var in the graph. That will always be true
             return [T.fill(node.inputs[in_idx],
                            T.constant(1.0, dtype=node.outputs[0].type.dtype))]
 
@@ -1591,13 +1598,13 @@ def local_useless_elemwise(node):
         elif node.op.scalar_op == theano.scalar.add and len(node.inputs) == 1:
             # No need to copy over any stack trace
             return [node.inputs[0]]
-        if (
-                node.op.scalar_op == theano.scalar.identity and
-                len(node.inputs) == 1
+            
+        elif (
+            node.op.scalar_op == theano.scalar.identity
+            and len(node.inputs) == 1
         ):
             return [node.inputs[0]]
-            
-        if (
+        elif (
             isinstance(node.op.scalar_op, scalar.AND)
             and len(node.inputs) == 2
         ):
@@ -1626,8 +1633,7 @@ def local_useless_elemwise(node):
                 and T.extract_constant(node.inputs[1]) == 0
             ):
                 return zeros_like(node, 0)
-                
-        if (
+        elif (
             isinstance(node.op.scalar_op, scalar.OR)
             and len(node.inputs) == 2
         ):
@@ -1655,8 +1661,7 @@ def local_useless_elemwise(node):
                 and T.extract_constant(node.inputs[1]) == 1
             ):
                 return ones_like(node, 0)
-                
-        if (
+        elif (
             isinstance(node.op.scalar_op, scalar.XOR)
             and len(node.inputs) == 2
         ):