====
>>> main.whiley
property nat(int x) -> (bool r):
   return x >= 0

property may_nat(int[] xs) -> (bool r):
   return some { i in 0..|xs| | nat(xs[i]) }

public export method test():
   assert may_nat([0])
   assert may_nat([-1,0])
   assert may_nat([0,-1])
   // assert may_nat([-2,-1,0])	
   // assert may_nat([-2,0,-1])
   // assert may_nat([0,-2,-1])
   // assert may_nat([0,1,-2,-1])
   // assert may_nat([0,-2,1,-1])
   // assert may_nat([0,-2,-1,1])
   // assert may_nat([-1,-2,-3])
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