todo modules:

• ring integers mod N
• generating function partial generators for some series
• make generic groups/rings work, bigints etc
• legendre (jacobi) symbol etc
• formal logic & formal language representation
• maybe graph stuff
• free groups & presentations?
• primes probably
  • off-memory cache? primes up to 8 billion are 1 GB
• quadratic integers, pythagorean triples etc
• maybe linear systems of modular equations
• combinatorics

in general don't want too many approximations