sub revcompl { # operates on all elements passed in
  my (@dna) = @_;
  foreach my $segment (@dna) {
    my $revcomp = reverse($segment);
    $revcomp =~ tr/ACGTacgt/TGCAtgca/;
    push @done, $revcomp;
  }
  return @done; # or reverse @done;  # what's best semantics?
}