predicate fzn_span_reif(var opt int: s0, var int: d0,
                        array[int] of var opt int: s,
                        array[int] of var int: d,
                        var bool: b) =
    b <-> (
      (occurs(s0) <-> exists(i in index_set(s))(occurs(s[i]))) /\ 
      s0 = min(s) /\
      (absent(s0) -> d0 = 0) /\
      s0 ~+ d0 = max([s[i] ~+ d[i] | i in index_set(s)])
    );