iOS Closures in Swift First Class Functions Higher Order Functions

In an extension to the String type, declare a function named transform. I can't figure it out. What's wrong?

Everything seems to be just fine but there an error: "... declare a function named transform".

functions.swift
// Enter your code below
extension String {
  func transform(argument: (String) -> String) -> String {
    return argument(self)

  }
}

I just forgot to omit an external label. That was an issue. No problems.