( ( mod-power
    =   base exponent modulus result
      .   !arg:(?base,?exponent,?modulus)
        & !exponent:~<0
        & 1:?result
        &   whl
          ' ( !exponent:>0
            &     ( (   mod$(!exponent.2):1
                      & mod$(!result*!base.!modulus):?result
                      & -1
                    | 0
                    )
                  + !exponent
                  )
                * 1/2
              : ?exponent
            & mod$(!base^2.!modulus):?base
            )
        & !result
    )
  & ( a
    = 2988348162058574136915891421498819466320163312926952423791023078876139
    )
  & ( b
    = 2351399303373464486466122544523690094744975233415544072992656881240319
    )
  & out$("last 40 digits = " mod-power$(!a,!b,10^40))
  )