code: @Beta
      public static <A, T, R> Function<T, R> sf(final Object mutex, final A a, final BiFunction<A, T, R> biFunction) {
          N.checkArgNotNull(mutex, "mutex");
          N.checkArgNotNull(biFunction, "biFunction");
  
          return new Function<T, R>() {
              @Override
              public R apply(T t) {
                  synchronized (mutex) {
                      return biFunction.apply(a, t);
                  }
              }
          };
      }
docs: Synchronized {@code Function}
  
  @param mutex to synchronized on
  @param u
  @param biFunction
  @return