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sub successor($k){
    $k+1;
}

sub cube($k) {
    $k**3;
}


#sub sub_cubf($n){
#    sum($n,successor,cube);
#}

sub natural($k) {
    $k;
}

sub pi($k) {
    8/($k*($k+2));
}
sub pi_term($k) {
    $k+4;
}
say sum_natural(1000);
say sum(1000,&natural,&successor);

say sum_pi(2e3);
say sum(2e3,&pi,&pi_term);

say sum_cube(10);
say sum(10,&cube,&successor);