1 - Integrate pdf-tools.cc with the rest of the code
2 - What if an input page specifies /Rotate?
4 - What should do modulo(half) with an odd number of tuples? Improve, or at least document.
5 - Document use of negative numbers in ranges
7 Ideas for further commands:
9 # Can be written as: mix(cat) { modulo(2){1}, modulo(2){2} }
13 # Send pages to multiple pipes and merge their results
15 cat # Concatenate results (default is interleave)
16 cycle # When one pipe ends earlier, cycle it from start
17 pad # When one pipe ends earlier, add blank pages