operator: fast-explain

    description:
        fast-explain[op] seq
        given a cause structure, and an input sequence, find the possible cause
        See here for why we would want to do this:
        Parsimonious Covering Theory with Causal Chaining and Ordering Constraints:
        https://github.com/garrettkatz/copct
        NB: copct.py code must be available for this function to work, so you will need to download it.
        Also note that copct is MIT licensed.

    examples:
        -- given this knowledge:
        cause |p> => |g> . |m> . |r>
        cause |t> => |p> . |p>
        cause |x> => |p> . |g>
        cause |z> => |r> . |p> 
        
        -- explain |g> . |m> . |r> . |g> . |m> . |r>:
        fast-explain[cause] ssplit |gmrgmr>
        
    see also:
        sexp, explain
        
    future:
        work out why in some cases it doesn't give the same results as "explain".