private static AssertionError fail(String msg,@Nullable Throwable cause){
  AssertionError e=new AssertionError(msg);
  if (cause != null)   e.initCause(cause);
  throw e;
}

private static AssertionError fail ( String msg , @ Nullable Throwable cause ) { AssertionError e = new AssertionError ( msg ) ; if ( cause != null ) e . initCause ( cause ) ; throw e ; }
throw assertion error with specified error message and initialized cause .
