optional parameter gc switches on gap costs (according to L1-model)

optional parameter ov means that overlaps between matches are allowed

optional parameter all means that all optimal chains are processed

If no parameter is given, compute local chains with maximums score.

If parameter is given, this must be a positive number optionally followed by the character b or p.

If only the number, say k, is given, this is the minimum score of the chains output.

If a number is followed by character b, then output all chains with the largest k scores.

If a number is followed by character p, then output all chains with scores at most k percent away from the best score.