PrefixSpan cannot find certain patterns

[gandreadis]

Moved from cafejojo/schaapi#78

Currently Prefix-Span won't generate all possible sub-sequences of a pattern, which means that it is not a correct implementation of the algorithm as put forward by Pei et al.

To illustrate, say we have as output sequence [node1, node2, node3, node4, node5]. In addition to the sequence itself, Prefix-Span will find all sub-sequences except:

• [node2, node3, node4, node5]
• [node2, node3, node4]
• [node3, node4, node5]

It will find entire sequences, but not all sub-sequences. So at the moment, it is sufficient to generate test cases. However this should eventually be fixed as it is not a proper implementation of the algorithm, and does not function as it should/how we expect it to.