TODO
====

In the next release:

  * a xor not a => true.
  * a eq not a => false.

  * Implement minimization (Quine-McCluskey, ESPRESSO).

  * Add higher orders / mixed case to logic_diff.

  * Check binding powers. Now Maxima can't parse
      "y eq not z nand t nor not x or not t"

  * Merge logic_simp with the basic Maxima simplifier (?)