function once(fn) {
  let called = false
  return function onceCaller() {
    if (!called) {
      called = true
      return fn.apply(this, arguments)
    }
  }
}
Wraps a function such that it will only be executed once.

@memberof Shim.prototype
@param {Function} fn - The function to wrap in an execution guard.
@returns {Function} A function which will execute `fn` at most once.