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  sub sum {
    my($term, $a, $next, $b) = @_;
    return 0 if $a > $b;
    return &$term($a) + sum($term, &$next($a), $next, $b);
  }

  sub sum {
    my($term, $a, $next, $b) = @_;
    return 0 if $a > $b;
    return $term->($a) + sum($term, $next->($a), $next, $b);
  }