infix operators, type 2: **

 ** :
    description:
        ket1 ** ket2
        multiply the values of ket1 and ket2
        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 multiply them component-wise

    examples:
        |3> ** |5>
            |15>

        |number: 3> ** |5>
            |number: 15>

        |number: 3> ** |page: 5>
            |>

        |2: 5> ** |3: 7>
            |6: 35>

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