This API is part of the Hack Standard Library, not HHVM itself.
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This function is available as Vec\diff() in Facebook's www repository.

Returns a new vec containing only the elements of the first Traversable that do not appear in any of the other ones


namespace HH\Lib\Vec;

function diff<Tv1 as arraykey, Tv2 as arraykey>(
  Traversable<Tv1> $first,
  Traversable<Tv2> $second,
  Traversable<Tv2> ...$rest,
): vec<Tv1>;

For vecs that contain non-arraykey elements, see Vec\diff_by().