A slight but significant speed up of the backtrack functions in
permbckt.g is possible:

replace Subgroup by G.operations.Subgroup (or similar) wherever it
is called inside the recursion.

I am doing some backtracking in a rather large degree (306936)
permutation group, and it is spending an irritatingly large amount
of time checking that elements passed to Subgroup are actually in
the parent group.

Actually I think this check should be disablable anyway, as it can
force construction of a Stabiliser chain where none is really needed.

Steve