~/src/github.com/darius/mccarthy-to-bryant (master)$ python
    >>> from nway_logic import *
    >>> A, B = Variable('A'), Variable('B')
    >>> A
    A
    >>> B
    B
    >>> const0 = Constant(0)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    NameError: name 'Constant' is not defined

    ~/src/github.com/darius/mccarthy-to-bryant (master)$ python
    Python 2.7.10 (default, Jul 13 2015, 12:05:58)
    [GCC 4.2.1 Compatible Apple LLVM 6.1.0 (clang-602.0.53)] on darwin
    Type "help", "copyright", "credits" or "license" for more information.
    >>> from dd import *
    >>> A, B = Variable('A'), Variable('B')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "dd.py", line 27, in Variable
        return build_node(rank, tuple(map(Constant, range(arity))))
      File "utils.py", line 7, in memoized
        result = memoized._memos[args] = f(*args)
      File "dd.py", line 34, in __init__
        for b in branches: assert rank < b.rank
    AssertionError
    >>>