A further safety element is constituted by the fact that said markings 10 are embedded in {HEAD ~ patterns} which constitute {TAIL ~ the normal safety printings} or patterns, indicated by 20, so that said markings cannot be easily perceived optically since, although they are present, they constitute an integral part of the safety pattern or printing 
constitute
A further safety element is constituted by the fact that said markings 10 are embedded in { HEAD ~ patterns } which constitute { TAIL ~ the normal safety printings } or patterns , indicated by 20 , so that said markings can not be easily perceived optically since , although they are present , they constitute an integral part of the safety pattern or printing
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH