Return expression which is the contradiction of `expr1` and `expr2`.

def Contradiction(expr1: Expression, expr2: Expression) -> Expression:
    """Return expression which is the contradiction of `expr1` and `expr2`."""
    expr = Disjunction(Conjunction(expr1, Negation(expr2)),
                       Conjunction(Negation(expr1), expr2))
    return ast.fix_missing_locations(expr)