the engagement structures 14 effectively mate with or in other words engage complementary structures defined in the cartridge interface 102 not shown to allow a secure fit between the cartridge 10 and the interface 102 moreover if the machine is designed to rotate or otherwise alter the position of cartridge 10 a driving motion of cartridge interface 102 should at least in part drive the motion or the cartridge 10 via the engagement structures 14 there may be provided within machine 10 other hardware for rotating the cartridge as well although fig2 shows the engagement structures 14 in the form of pairs of concave surfaces variants suitable for the invention can be imagined the engagement structures 14 can be concave or convex or include convex and concave portions or within a single cartridge 10 there may be defined both concave and convex engagement structures indeed in order to permit the use of the engagement structures for driving the rotation of the cartridge 10 the engagement structures could include for instance high friction or even magnetic elements which effectively engage with complementary structures in the cartridge interface 102 fig3 and 4 are end on elevational views of two respective types of cartridges 10 illustrating another aspect of the present invention the cartridges.