isProof: Move 'isModusPonens :: Wff -> Wff -> Wff -> Bool' to the top level
isProof: Move 'isSubstitution  :: Wff -> Wff -> Wff -> Bool' to the top level
TestTheorem: Add more test cases

One of the new test cases for isSubstitution exposes a bug.
isSubstitution: Fix bug and add another test
TestTheorem: Fix testIsProof

Fix parse error in a wff used in testIsProof