MiniZinc: evaluation error: 
  roots_bad.mzn:8:
  in call 'roots'
  /Users/tack/Programming/libmzn/tests/../share/minizinc/std/roots.mzn:6:
  in call 'assert'
  Assertion failed: roots: upper bound of 's' must be a subset of the index set of 'x'