The arguments to C<traverse_sequences> are the two sequences to
traverse, and a callback which specifies the callback functions, like
this:

  traverse_sequences( \@seq1, \@seq2,
                     { MATCH => $callback_1,
                       DISCARD_A => $callback_2,
                       DISCARD_B => $callback_3,
                     } );

Callbacks are invoked with at least the indices of the two arrows as
their arguments.  They are not expected to return any values.  If a
callback is omitted from the table, it is not called.