sequence function: find-path-between

    description:
        find-path-between(ket, sp)
        find the path between the ket, and the given superposition
        currently, potentially quite slow! But this is fixable.
      
    examples:
        load fred-sam-friends.sw
        find-inverse[friends]
        find-path-between(|Fred>, |Sam>)
            |op: friends> . |op: inverse-friends>
        
        find-path-between(|Fred>, |Julie>)
            |op: friends> . |op: inverse-friends> . |op: friends>

        
        learn-map[10,10]
        find-path-between(|grid: 1: 1>, |grid: 3: 7>)
            |op: E> . |op: E> . |op: E> . |op: E> . |op: SE> . |op: SE>

    
        load george.sw
        find (*) #=> find-path-between(|person: George>, |_self>)
        find |person: Andrew>
            |op: friends>

        find (|person: Sarah> + |person: David> + |person: Frank>)
            |op: family>

        find (|person: Emily> + |person: Fred>)
            |op: family-and-friends>

        find (|person: Frank> + |person: Emily>)
            |op: siblings>

    future:
        optimize it with Dijkstra's algorithm
        (since current algorithm is horrible!)
        make it work with path-ways between sequences too.

    see also:
        finding-a-path-between-early-us-presidents worked example