title: "contextual closure type expects 2 arguments" error when using reduce in Swift 4
body: <p>The following code compiles in Swift 3</p>
  
  <pre><code>extension Array where Element: Equatable {
      var removeDuplicate: [Element] {
          return reduce([]){ $0.0.contains($0.1) ? $0.0 : $0.0 + [$0.1] }
      }
  }
  </code></pre>
  
  <p>but produces the error</p>
  
  <blockquote>
    <p>error: contextual closure type '(_, _) -> _' expects 2 arguments, but 1 was used in closure body</p>
  </blockquote>
  
  <p>in Swift 4. How to convert this code to be compiled in Swift 4?</p>