my ($pos,$strand,$phase) = @_;
  $strand ||= +1;
  $phase  ||= 0;
  my $frame = $strand >= 0 
    ? ($pos - $phase - 1) % 3
    : (1 - $pos - $phase) % 3;
  my $offset = -$phase % 3;
  $offset   *= -1 if $strand < 0;
  return wantarray ? ($frame,$offset) : $frame;
}


1;