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