Advanced structured concurrency: "@" ("Alt") operator

      Expr1 @ Expr2 @ ... @ ExprN

• Executes expressions 1..N in parallel.
• As soon as one of the expressions finishes evaluating, its result is returned and the other pending expressions are cancelled.
• Idea of 'structured concurrency' borrowed from Austin University's Orc language.
• Most obvious use is for exploring alternatives:

      function get_cnn_feed() { ... }
      function get_bbc_feed() { ... }
      function send_mail() { ... }

      send_mail(get_cnn_feed() @ get_bbc_feed(), 
                "alex@croczilla.com");

• But ... many concurrency problems fit into this pattern.
• Useful for implementing timeouts, user cancellation, reliability, ...