operator: rel-kets

    description:
        rel-kets[op]
        return all relevant kets that have op specified
        if op is * then return everything that has a literal operator specified for it
        NB: rel-kets[op] does not return kets that are only specified on the right hand side of a learn rule 
        
        rel-kets[op1,op2, ..., opn]
        return all relevant kets that have op1, ... , opn specified
        
        rel-kets[op1, ... , opn] sp
        return all relevant kets, in the given superposition, that have op1, ... , opn specified

    examples:
        load fred-sam-friends.sw
        rel-kets[friends]
            |Fred> + |Sam>
        
        age |Sam> => |47>
        rel-kets[friends, age]
            |Sam>
        
        load family.sw
        rel-kets[*]
            |context> + |sally> + |erica> + |trude> + |peter> + |tom> + |sara> + |sam> + |ruth> + |mike> + |gina> + |mary> + |mark>
        
        load family-relations.sw
        such-that[is-a-female] rel-kets[*]
            |sally> + |erica> + |trude> + |sara> + |ruth> + |gina> + |mary>

        such-that[is-an-uncle] rel-kets[*]
            |peter>

        such-that[have-a-sister] rel-kets[*]
            |sally> + |erica>
            
        such-that[have-a-sister, is-a-teenager] rel-kets[*]
            |sally>

        such-that[have-a-brother] rel-kets[*]
            |peter> + |tom>

         such-that[have-a-wife] rel-kets[*]
            |tom> + |sam> + |mike> + |mark>

    see also:
        such-that
        
    TODO:
        fix sequence behaviour
        maybe implement all-kets which returns even kets that are only specified on the right of a learn rule