 
 extension RxAssertion {
   public func within(_ timeRange: Range<TestTime>) -> RxAssertion<O> {
-    var copy = self
-    copy.timeRange = timeRange
-    return copy
+    self.timeRange = timeRange
+    return self
   }
 
   public func since(_ timeSince: TestTime) -> RxAssertion<O> {


extension RxAssertion {
  public func not() -> RxAssertion<O> {
    var copy = self
    copy.isNot = !self.isNot
    return copy
  }
}



extension RxAssertion {
  public func filterNext() -> RxAssertion<O> {
    self.shouldFilterNext = true
    return self
  }
}

