original.name="String_Valid_1"
======
>>> main.whiley


type nat is (int x) where x >= 0

function f(nat pos, int[] input) -> bool|null:
    if pos >= |input|:
        return null
    else:
        bool flag = input[pos] == 'O'
        return flag

public export method test():
    assume f(0, "Ox") == true
    assume f(0, "1x") == false
    assume f(1, "O") == null

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