infix operators, type 1: -

 - :
        ket1 - ket2
        the infix operator for ket subtraction
        If the label of ket1 is the same as the label of ket2, then subtract their coefficients
        If the labels are distinct, then we have a superposition
        Where a superposition is the sum of one or more kets
        Note, the identity element for ket addition is the empty ket |>
        Ie, sp + |> == |> + sp == sp, for any superposition sp
        Note that combined with the drop operator, subtraction can be seen as a kind of inhibition

        -- taking two apples from five apples:
        5|apple> - 2|apple>

        -- taking 5 apples from 7 oranges:
        -- we can't subtract them directly, they have different labels, so return a superposition:
        7|orange> - 5|apple>
            7|orange> - 5|apple>

    see also:
        drop, + , _ , __ , :_ , .