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

import RxSwift
import RxTest

public struct RxAssertion<O: ObservableConvertibleType> {
  unowned let expectation: RxExpectation
  let asserter: Asserter
  let source: O


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

  public func since(_ timeSince: TestTime) -> RxAssertion<O> {
