BidirectionalCollection
protocol BidirectionalCollection : Collection- 
                  
                  Returns elements of this collection joined using a separator between each two elements. Last separator can be a different one. Note Useful to create strings like: One, two, three and four .DeclarationSwift public func joined(separator: String, lastElementSeparator: String) -> StringParametersseparatorSeparator to be used between each two elements but the last pair. lastElementSeparatorSeparator to be used between last two elements. Return ValueString with this collection’s elements joined. 
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           BidirectionalCollection Extension Reference
      BidirectionalCollection Extension Reference