Path matching issue

Hi all,

I'm having an issue with matching paths of different lengths. 

What I'd like to do is have it so that all branches match such that:

{a;a;x} matches with {a;a}

{a;b;x} matches with {a;b}

{b;a;x} matches with {b;a}

etc...

At the moment it seems to be that for the first two branches, i.e. {0;0;0} and {0;0;1}, instead of them matching with {0;0}, the first 0 is being ignored and they are matched with {0;0} and {0;1}.

How can I fix this without exploding the tree and grafting each of the data sets?