fix edge case in block concatenation

[mmolari]

There was an error happening when merging transitive edges. This was due to the following edge-case:

• consider the concatenation of two blocks b1 and b2.
• b1 has an insertion at the end of the block, at position L1
• b2 has an insertion at the beginning of the block, at position 0
• when concatenating them into a new block b1+b2 there will be two insertions at position L1, the boundary between the two original blocks. And there is an ambiguity on which one to apply first.

To fix this at the moment of merging the two sets of insertions, if two have the same position I join them in a single insertion, appending the second one to the first one.