ket fn: log

log:
    description:
        log ket
        take the natural log of the value in the ket label
        while preserving the ket label category
        if the value is not a number, then return the ket unchanged
        also, we preserve the ket coefficient

    examples:
        -- log of a simple number:
        log |2.71828>
            |1>

        -- log of a number with a category, in this case "number"
        log |number: 5.3>
            |number: 1.66771>

        -- attempted log of a non number:
        log |alpha>
            |alpha>

    see also:
        floor, ceiling