infix operators, type 2: --

 -- :
    description:
        ket1 -- ket2
        subtract the value of ket2 from ket1
        we do our best to preserve categories
        if one ket has a category, and the other doesn't then the result has that category
        if ket1 and ket2 have different categories then return the empty ket |>
        if ket1 and ket2 are 2D, then subtract them component-wise

    examples:
        |3> -- |5>
            |-2>

        |number: 3> -- |5>
            |number: -2>

        |number: 3> -- |page: 5>
            |>

        |2: 5> -- |3: 7>
            |-1: -2>

    see also:
        arithmetic, ++ , ** , // , %% , ^^