All Of:                                    
                                                                                                                                                                                                                                                                                                                                                     
                                                           preconditions                                                                                                                                                                                                                                  1.                                   prec 1
                                                                                                                                                                                                                                                                                                          2.                                   prec 2
                                                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                     
                                                                                                                              do A                                                                                                                                                                                                                   
                                                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                     
                                                         yes                                         success?                                                                                                                 no                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                     
                                                                                                                                          no                                                                                                                                                                                                         
                                A is OK?                                                                                                                                                                  do C                                                                                                                                       
                                                                                                                                                                                                                                                                                                                                                     
                                                                                   yes                                                                                                                                                                                                                                                               
                                                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                     
                                                 do B