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Hash#filter! is an alias for Hash#select!.
Returns self, whose entries are those for which the block returns a truthy value:
h = {foo: 0, bar: 1, baz: 2}
h.select! {|key, value| value < 2 }  => {:foo=>0, :bar=>1}
Returns nil if no entries were removed.
Returns a new Enumerator if no block given:
h = {foo: 0, bar: 1, baz: 2}
e = h.select!  # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:select!>
e.each { |key, value| value < 2 } # => {:foo=>0, :bar=>1}
 
               static VALUE
rb_hash_select_bang(VALUE hash)
{
    st_index_t n;

    RETURN_SIZED_ENUMERATOR(hash, 0, 0, hash_enum_size);
    rb_hash_modify_check(hash);
    n = RHASH_SIZE(hash);
    if (!n) return Qnil;
    rb_hash_foreach(hash, keep_if_i, hash);
    if (n == RHASH_SIZE(hash)) return Qnil;
    return hash;
}
            

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