ket fn: extract-tail

        extract-tail ket
        categories are separated by ": "
        with the most general category on the left, and the most specific on the right
        for example |animal: mammal: dog>
        has the most general category "animal", then the category "mammal" and then finally the "value" dog
        This list of categories and the value can be considered a type of list
        extract-tail returns the tail of this list
        Ie, with the first category removed

        -- if there is no category, then return the empty ket:
        extract-tail |dog>

        extract-tail |animal: mammal: dog>
            |mammal: dog>

        -- an abstract example:
        extract-tail |a: b: c: d: e: f>
            |b: c: d: e: f>

    see also:
        extract-category, extract-value, extract-head