operator: to-lower

    description:
        to-lower |x>
        set all characters to lower case 

        to-lower[i1, i2, ... , in] |x>
        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