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each_key 

        # Hash.each_key

(from ruby core)
---
    hash.each_key {|key| ... } -> self
    hash.each_key -> new_enumerator

---

Calls the given block with each key; returns `self`:
    h = {foo: 0, bar: 1, baz: 2}
    h.each_key {|key| puts key }  # => {:foo=>0, :bar=>1, :baz=>2}

Output:
    foo
    bar
    baz

Returns a new Enumerator if no block given:
    h = {foo: 0, bar: 1, baz: 2}
    e = h.each_key # => #<Enumerator: {:foo=>0, :bar=>1, :baz=>2}:each_key>
    h1 = e.each {|key| puts key }
    h1 # => {:foo=>0, :bar=>1, :baz=>2}

Output:
    foo
    bar
    baz

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