Checking for the presence of keys in a hash: fast
h = { :a => 1, :b => 2 }
h.key?(:a) # true: :a is a key in h
h.has_key?(:b) # true: has_key? is a synonym for key?
h.include?(:c) # false: include? is another synonym
h.member?(:d) # false: member? is yet another synonym
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