public String toString() {
            return fallback.toString();
          }
        };
    return derive(future.catchingAsync(exceptionType, asyncFunction, executor));
  }

  /**
   * Marks this step as the last step in the {@code ClosingFuture} pipeline.
   *
   * <p>The returned {@link Future} is completed when the pipeline's computation completes, or when
   * the pipeline is cancelled.
   *

<p>
The grammar is compact and simple to parse, allowing for easy analysis
by automatic tools such as integrated development environments.
</p>

<h2 id="Notation">Notation</h2>
<p>
The syntax is specified using Extended Backus-Naur Form (EBNF):
</p>

<pre class="grammar">
Production  = production_name "=" [ Expression ] "." .
Expression  = Alternative { "|" Alternative } .
Alternative = Term { Term } .