Return an array containing all the keys in a hash with keys:
zip = { 1 => "One", 2 => "Two", 3 => "Three",
4 => "Four", 5 => "Five", 6 => "Six", 7 =>
"Seven", 8 => "Eight", 9 => "Eight" }
zip.keys # => [8, 3, 6, 9, 5, 2, 1, 4, 7]
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