Show how to add a mutating function for RangeReplaceableCollectionType that appends items only if they are not already in the collection.

Collections+Additions.swift

extension RangeReplaceableCollectionType where Generator.Element : Equatable {
    mutating func appendDistinct(object : Generator.Element) {
        if !self.contains(object) {
            self.append(object)
        }
    }
}

// A derivative solution using filter instead of contains
extension RangeReplaceableCollectionType where Generator.Element : Equatable {
    mutating func appendDistinct(object : Generator.Element) {
        let filtered = self.filter( { $0 == object } )
        if filtered.first == nil {
            self.append(object)
        }
    }
}