# chain_lines takes a list of coderefs
  # The first sub in the chain is called with no input; subsequent ones are called
  # with $_ set to the line from the previous sub; the iterator returns one line of
  # output from the last sub in the chain. If a sub yields undef, processing
  # restarts at the first sub. When the first sub yields undef, the iterator is done.
  my $i = chain_lines sub { <$in> },
                      sub { /^42/ ? $_ : undef },    # grep
                      sub { join "\t", (split /\t/)[1,3,8] };
  # all_lines expands the iterator into a list of lines
  sort all_lines($i);