[Bug] [BE] Randomness in rule generation

[colinthebomb1]

Issue

In rule generalization, there are some cases where the output is somewhat random, making it impossible to create concrete tests for certain query pairs.

Example

In the engine spreadsheet, ID 2 has been shown to produce several outputs.
FROM <x1> WHERE <x2> OR EXISTS (SELECT <x3> FROM <x4> WHERE <x5> IN (<x6>, <x7>, <x8>, <x9>) AND <<x10>>)
and
FROM <x1> WHERE <x2> OR EXISTS (SELECT <<x3>> FROM <x4> WHERE <x5> IN (<x6>, <x7>, <x8>, <x9>) AND <<x10>>)

This may be related to issue #80