A logical problem with SplitCommonSuffices and MergeCommonSuffices

@Sheila @valentin @depristo
For example ：
A+x -> y (A+x,y is a point)
B+x -> y
after SplitCommonSuffices
A -> x -> y (A,B,x,y is a point)
B -> x -> y
after MergeCommonSuffices
A -> x => A+x
B -> x => B+x
then after SplitCommonSuffices after MergeCommonSuffices ...
of course some time after SplitCommonSuffices SeqVertex id is bigger than before but this may occured in unintended(unmind) circumstances.