exercism/xpython — pull request #136 by mmakaay (2014-10-21 13:22:00+00:00)
Title: Added test_is_valid_can_be_called_repeatedly() to the test set.

This test checks if is_valid() can be called multiple times.
Many implementations failed for this, because a list of digits
that was created at init time, got mangled by the Luhn
transformation, causing the second call to is_valid() to
work with different input data.