[ dup upper != ]              is lower? ( c --> b )

  [ dup lower != ]              is upper? ( c --> b )

  [ swap $ "" unrot witheach
      [ dup lower? iff
          [ over +
            dup lower? not if
              [ 26 - ] ]
        else
          [ dup upper? if
            [ over +
              dup upper? not if
                [ 26 - ] ] ]
        rot swap join swap ]
    drop ]                      is caesar ( $ --> $ )

  [ 26 swap - ]                 is decode ( n --> n )

  $ "Fere libenter homines id quod volunt credunt."
  dup              echo$ cr
  23 caesar dup    echo$ cr
  23 decode caesar echo$ cr