of

Return true if x is a equal to any of ys. TODO Make ys lazy if any of ys is a delegate. TODO Reuse among

  1. bool of(S x, T ys)
    pure
    bool
    of
    (
    S
    T...
    )
    (
    in S x
    ,
    in T ys
    )
    if (
    ys.length >= 1 &&
    )
  2. bool of(E x, R ys)

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