with javascript_semantics
include mpfr.e
procedure mpz_mod_exp(mpz base, exponent, modulus, result)
    if mpz_cmp_si(exponent,1)=0 then
        mpz_set(result,base)
    else
        mpz exp_copy = mpz_init_set(exponent)
        bool bOdd = mpz_odd(exp_copy)
        if bOdd then
            mpz_sub_ui(exp_copy,exp_copy,1)
        end if
        mpz_fdiv_q_2exp(exp_copy,exp_copy,1)
        mpz_mod_exp(base,exp_copy,modulus,result)
        exp_copy = mpz_free(exp_copy)
        mpz_mul(result,result,result)
        if bOdd then
            mpz_mul(result,result,base)
        end if
    end if
    mpz_mod(result,result,modulus)
end procedure

mpz base     = mpz_init("2988348162058574136915891421498819466320163312926952423791023078876139"),
    exponent = mpz_init("2351399303373464486466122544523690094744975233415544072992656881240319"),
    modulus  = mpz_init("1e40"),
    result   = mpz_init()

mpz_mod_exp(base,exponent,modulus,result)
?mpz_get_str(result)

-- check against the builtin:
mpz_powm(result,base,exponent,modulus)
?mpz_get_str(result)