 }
 
 public func +<
-    C : _ExtensibleCollectionType,
+    C : ExtensibleCollectionType,
     S : CollectionType
     where S.Generator.Element == C.Generator.Element
 >(var lhs: C, rhs: S) -> C {

}

public func +<
    EC1 : _ExtensibleCollectionType,
    EC2 : _ExtensibleCollectionType
    where EC1.Generator.Element == EC2.Generator.Element
>(var lhs: EC1, rhs: EC2) -> EC1 {
  // FIXME: what if lhs is a reference type?  This will mutate it.

//
//===----------------------------------------------------------------------===//

/// A collection type that can be efficiently appended-to.
public protocol ExtensibleCollectionType : CollectionType {
  /// Create an empty instance.
  init()

