compound ket fn: to-lower

to-lower:
    description:
        to-lower ket
        set all characters to lower case

        to-lower[i1, i2, ..., in] ket
        change i'th characters to lower case

    examples:
        to-lower |FRED>
            |fred>

        to-lower[1] |FRED>
            |fRED>

        to-lower[1,3,5] |ABCDEFG>
            |aBcDeFG>

    see also:
        to-upper