Detect dead variables

[cbettinger]

The modified DIMACS file format contains a list of dead variables. This is list presumed at the moment.

Dead variables can be detected by repeated SAT solving the problem p (containing variables v1-vn), considering an additional conjecture in each run: If p AND vx is UNSAT then vx is dead (given that p is SAT).

An additional mode should be implemented that detects and outputs a list of dead variables.