code: public RetryerBuilder<V> withBlockStrategy(@Nonnull BlockStrategy blockStrategy) throws IllegalStateException {
          Preconditions.checkNotNull(blockStrategy, "blockStrategy may not be null");
          Preconditions.checkState(this.blockStrategy == null, "a block strategy has already been set %s", this.blockStrategy);
          this.blockStrategy = blockStrategy;
          return this;
      }
docs: Sets the block strategy used to decide how to block between retry attempts. The default strategy is to use Thread#sleep().
  
  @param blockStrategy the strategy used to decide how to block between retry attempts
  @return <code>this</code>
  @throws IllegalStateException if a block strategy has already been set.