misc: |*>

        see label descent.
        this special ket is used if we want to define an operator that is defined with respect to all kets
        instead of being defined with respect to a single specific ket
        so, in some sense it is a function, but be aware it is applied ket-wise
        so if you apply it to a superposition or sequence, it will be applied to each sub-ket in turn
        this makes it essentially linear, but not quite, depending on the details of the operator
        it is usually, but not always, defined with respect to stored or memoizing rules

        -- a quick demonstration of its "linearity":
        -- just define an operator that adds the "op" category to a given ket:
        op |*> #=> |op> :_ |_self>

        -- now invoke it:
        -- first to a superposition:
        op split |abcde>
            |op: a> + |op: b> + |op: c> + |op: d> + |op: e>

        -- now to a sequence:
        op ssplit |abcde>
            |op: a> . |op: b> . |op: c> . |op: d> . |op: e>

        -- NB: there is an unsolved bug, where the coefficient of the input ket is squared!
        -- so for example:
        op rank split |abcde>
            |op: a> + 4|op: b> + 9|op: c> + 16|op: d> + 25|op: e>

    see also:
        label descent, |category: *>, |_self>, #=> , !=>