Use explicit logic for pruning

Also a speedup since we don't need to recalculate SEE
for extensions...as it already determined to be positive.

Results for 12 tests for each version:

        Base      Test      Diff
Mean    2132395   2191002   -58607
StDev   128058    85917     134239
p-value: 0.669
speedup: 0.027

Non functional change.