A before-change code read section included in a program change management apparatus reads out a before-change code stored in a before-change code storage section. Once the variable is located, the information contained within the identified hypertext element may be used in other applications. The determined ordering can then be used to generate display data where individual interactions are displayed as lines connecting co-ordinates corresponding to the positions in the determined ordering of interacting elements and the time value associated with the interaction. A support film having high UV transmittance is attached to the partially cured high polymer. A method of perceptibly watermarking the material comprises the steps of: changing representations of the said numbers according to an invertible algorithm without changing the number of bits in the said numbers; and encoding the changed n bit numbers to effect compression encoding of the material.