The invention provides compounds of formula wherein n, A, R1, R2 and R3 are as defined in the description, and the preparation thereof. A computer program product is also disclosed. The cylindrical wall includes an inner liner and an outer liner concentric to the inner liner. The architecture comprises an allocator for allocating incoming requests from the master device to a target slave device and an optimiser for each slave device. The processor may receive the item from the content provider and provide the interface to the users.