of

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

  1. bool of(S x, T ys)
  2. bool of(E x, R ys)
    pure
    bool
    of
    (
    E
    R
    )
    (
    E x
    ,
    R ys
    )
    if (
    isIterable!R
    )

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