- Integrate pdf-tools.cc with the rest of the code
- What if an input page specifies /Rotate?
- "-f" switch
- What should do modulo(half) with an odd number of tuples? Improve, or at least document.
- Document use of negative numbers in ranges

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