Issue with `is_equivalent` using different `Licensing` instances

[tdruez]

While this first code works fine:

from license_expression import Licensing

expression = 'gpl-2.0 AND zlib'
licensing = Licensing()
parsed1 = licensing.parse(expression)
parsed2 = licensing.parse(expression)
assert Licensing().is_equivalent(parsed1, parsed2)

This second fails:

expression = 'gpl-2.0 AND zlib'
parsed1 = Licensing().parse(expression)
parsed2 = Licensing().parse(expression)
assert Licensing().is_equivalent(parsed1, parsed2)

It seems to be related to the operator since this last one works as well:

expression = 'gpl-2.0'
parsed1 = Licensing().parse(expression)
parsed2 = Licensing().parse(expression)
assert Licensing().is_equivalent(parsed1, parsed2)

[pombredanne]

@tdruez Thank you. The difference between case 1 and 2 is that you are using two Licensing object instances in case 2 rather than the same Licensing object in case one. Equivalence is made within the confine of a Licensing object instance. So I guess that the resolution can be one of these:

1. workaround: you use a single licensing object
2. whenever a check is requested for two expressions, check that both come from the same licensing. If not ensure that we unparse/reparse one of the two to be from the same Licensing as the other
3. add support for doing cross Licensing expression comparisons

[pombredanne]

@tdruez this not-yet-test commit fixes things up from what I can see. The changes are only in boolean.py in https://github.com/bastikr/boolean.py/tree/proper-equivalence
Not yet released on Pypi either

[tdruez]

@pombredanne thanks++ for the fix!