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h = {:a=>1, :b=>2, :c=>3, :d=>"four"}
h.reject! {|k,v| v.is_a? String } # => {:a=>1, :b=>2, :c=>3 }
h.delete_if {|k,v| k.to_s < 'b' } # => {:b=>2, :c=>3 }
h.reject! {|k,v| k.to_s < 'b' } # => nil: no change
h.delete_if {|k,v| k.to_s < 'b' } # => {:b=>2, :c=>3 }: unchanged hash
h.reject {|k,v| true } # => {}: h is unchanged
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