This code introduces a check to prevent infinite loop cases. In the test case added in 'test_unsolvable', the software looped infinitely rather than raising an error. I tested this and found that the error was occurring after the cost_matrix had been padded.

An infinite loop occurs where all remaining uncovered values are DISALLOWED. This identifies the case and raises an UnsolvableMatrix exception. I have not written a test for this as my current reproducable example is a 500 by 400 matrix.