Changes between epics 3.13 Beta 3 and 3.13 Beta 4 
**** API Changes ****
o The canonical PV name is returned from caServer::pvExistTest()
in the supplied buffer and not in a gdd data descriptor. See
"casdef.h".
old API:
        //
        // pvExistTest()
        //
        // The server tool is encouraged to accept multiple PV name
        // aliases for the same PV here. However, a unique canonical name
        // must be selected for each PV.
        //
        // returns S_casApp_success and fills in canonicalPVName
        // if the PV is in this server tool
        //
        // returns S_casApp_pvNotFound if the PV does not exist in
        // the server tool
        //
        // The server tool returns the unique canonical name for
        // the pv into the gdd. A gdd is used here becuase this
        // operation is allowed to complete asynchronously.
        //
        virtual caStatus pvExistTest (const casCtx &ctx, const char *pPVName,
                                gdd &canonicalPVName) = 0;
new API:
        //
        // pvExistTest()
        //
        // The server tool is encouraged to accept multiple PV name
        // aliases for the same PV here. However, one unique canonical name
        // must be selected by the srever tool and returned to the
        // server lib for each PV. The server will use this canonical
        // name to prevent internal duplication of data structures for
        // process variables that have multiple aliases.
        //
        // o returns S_casApp_success and a valid canonical name string
        // when the PV is in this server tool
        //
        // o returns S_casApp_pvNotFound if the PV does not exist in
        // the server tool
        //
        // Examples:
        // caServerXXX::pvExistTest(const casCtx &ctx, const char *pPVName)
        // {
        //      return pvExistReturn(S_casApp_success, pPVName); // common
        //      return pvExistReturn(S_casApp_pvNotFound); // no PV by that name
        //
        //      char pName[9] = "myPVName";
        //      return pvExistReturn(S_casApp_success, pName); // also common
        //      return pvExistReturn(S_casApp_asyncCompletion); // not now
        // }
        //
        virtual pvExistReturn pvExistTest (const casCtx &ctx,
				const char *pPVName)=0;

o The server tool must now use one of class casAsyncReadIO, casAsyncWriteIO, or
casAsyncPVExistIO in place of casAsyncIO. See "casdef.h".