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keep_if 

        # Hash.keep_if

(from ruby core)
---
    hash.keep_if {|key, value| ... } -> self
    hash.keep_if -> new_enumerator

---

Calls the block for each key-value pair; retains the entry if the block
returns a truthy value; otherwise deletes the entry; returns `self`.
    h = {foo: 0, bar: 1, baz: 2}
    h.keep_if { |key, value| key.start_with?('b') } # => {:bar=>1, :baz=>2}

Returns a new Enumerator if no block given:
    h = {foo: 0, bar: 1, baz: 2}
    e = h.keep_if # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:keep_if>
    e.each { |key, value| key.start_with?('b') } # => {:bar=>1, :baz=>2}

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