Fix bugs in diff() and simp(). Add a smoke test.

Author: darius

logic.py

@@ -977,7 +977,7 @@ def diff(y, x):
         u, op, v = y.args[0], y.op, y.args[-1]
         if op == '+':
             return diff(u, x) + diff(v, x)
-        elif op == '-' and len(args) == 1:
+        elif op == '-' and len(y.args) == 1:
             return -diff(u, x)
         elif op == '-':
             return diff(u, x) - diff(v, x)
@@ -998,7 +998,7 @@ def diff(y, x):
 
 def simp(x):
     "Simplify the expression x."
-    if not x.args:
+    if isnumber(x) or not x.args:
         return x
     args = list(map(simp, x.args))
     u, op, v = args[0], x.op, args[-1]

tests/test_logic.py

@@ -173,6 +173,9 @@ def test_ask(query, kb=None):
     assert repr(test_ask('Rabbit(x)')) == '[{x: MrsRabbit}, {x: Pete}]'
     assert repr(test_ask('Criminal(x)', crime_kb)) == '[{x: West}]'
 
+def test_d():
+    assert d(x*x - x, x) == 2*x - 1
+
 def test_WalkSAT():
     def check_SAT(clauses, single_solution = {}):
         # Make sure the solution is correct if it is returned by WalkSat