Commands to add
===============

- Commands that change streams:
  stream-foreach
  stream-reverse

- Maybe:
  layer -m|--mode=min|max|median|or|and|xor|diff|add|xadd|sub|xsub|mul|div file...


Other things
============

- Add a test for every command. The test infrastructure is already finished,
  including automatic use of valgrind if available.
