define product = proc (m n)
                          (productFork m n
                                       newref(1) newref(1)
                                       newref(false) newref(false))
       define productFork = proc (m n
                                  refProduct1 refProduct2
                                  refDone1 refDone2)
                              begin choose((productLoop m
                                                        (avg m n)
                                                        refProduct1
                                                        refDone1),
                                           (productLoop (avg m n)
                                                        n
                                                        refProduct2
                                                        refDone2));
                                    (productJoin refProduct1 refProduct2
                                                 refDone1 refDone2)
                              end
       define productJoin = proc (refProduct1 refProduct2
                                  refDone1 refDone2)
                              if deref(refDone1)
                                then if deref(refDone2)
                                       then begin
                                              *(deref(refProduct1),
                                                deref(refProduct2))
                                            end
                                       else begin
                                              fail;
                                              (productJoin refProduct1
                                                           refProduct2
                                                           refDone1
                                                           refDone2)
                                            end
                                else (productJoin refProduct1
                                                  refProduct2
                                                  refDone1
                                                  refDone2)
       define productLoop = proc (m n refProduct refDone)
                              if <(m,n)
                                then begin setref(refProduct,
                                                  *(m,deref(refProduct)));
                                           (productLoop +(m,1)
                                                        n
                                                        refProduct
                                                        refDone)
                                     end
                                else setref(refDone,true)
       define avg = proc (m n) /(+(m,n),2)
       (product 1 21)