=====
>>> main.whiley
property sum(int[] xs, int i) -> (int r):
   if i < 0 || i >= |xs|:
      return 0
   else:
      return xs[i] + sum(xs,i+1)

public export method test():
   assert sum([],0) == 0
   assert sum([1],0) == 1
   assert sum([1,2],0) == 3
   assert sum([1,2,3],0) == 6
   assert sum([1,2,3],1) == 5
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