- Integrate pdf-tools.cc with the rest of the code
- "-f" switch
- What should do modulo(half) with an odd number of tuples? Improve, or at least document.
- Modulo with last tuple incomplete: sometimes, not padding the last tuple can be useful
  (e.g., "flip every other page")
- Document use of negative numbers in ranges
- Allow use of page labels to refer to pages
- Better range syntax for "from N to end" or "the whole document"
- nup: allow stacking ("1 3 5 7" "2 4 6 8")
- support "paper=a4r" for rotated A4

Ideas for further commands:

# Can be written as: mix(cat) { modulo(2){1}, modulo(2){2} }
duplex
	long-edge

# Send pages to multiple pipes and merge their results
mix { ..., ..., ...}
	cat		# Concatenate results (default is interleave)
	cycle		# When one pipe ends earlier, cycle it from start
	pad		# When one pipe ends earlier, add blank pages