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keep_if 

        # Array.keep_if

(from ruby core)
---
    array.keep_if {|element| ... } -> self
    array.keep_if -> new_enumeration

---

Retains those elements for which the block returns a truthy value;
deletes all other elements; returns `self`:
    a = [:foo, 'bar', 2, :bam]
    a.keep_if {|element| element.to_s.start_with?('b') } # => ["bar", :bam]

Returns a new Enumerator if no block given:
    a = [:foo, 'bar', 2, :bam]
    a.keep_if # => #<Enumerator: [:foo, "bar", 2, :bam]:keep_if>

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