cl-join
           an enhanced version of the Unix `join' coreutil

- any number of input files
- numerically sorted join fields
- specify single non-key field to include in output
- include unpaired lines with empty

  $ join -h
  Usage: join [OPTIONS...] FILES...

  For each pair of input lines with identical join fields, write a
  line to standard output.  The default join field is the first,
  delimited by whitespace.  (TODO: When a single FILE is -, read
  standard input.)

  Options:
   -n,--numbers ------- keys sorted according to numerical values
   -e,--empty EMPTY --- replace missing input fields with EMPTY
                        includes unpaired lines
   -i,--ignore-case --- ignore case when comparing fields
   -j,--join FIELD ---- join on this FIELD of each file
   -v,--value FIELD --- only include FIELD of each file in output
   -t,--sep REGEX ----- use REGEX as input field separator
   -o,--output CHAR --- use CHAR as output field separator
   TODO: -N,--fieldN FIELD -- join on this FIELD of file N
   --header ----------- treat the first line in each file as field
                        headers, print them without pairing them