infix operators, type 2: <=

 <= :
    description:
        ket1 <= ket2
        test whether ket1 is less than or equal to ket2
        if either of the kets are not numbers, then return the empty ket
        if the categories for the kets are different, again, we return the empty ket

    examples:
        |5> <= |8>
            |yes>

        |number: 3> <= |number: 1>
            |no>

        |7> <= |fish>
            |>

        |number: 11> <= |page: 7>
            |>

    see also:
         == , != , >= , > , <