Well-formed Nets (WN) structural analysis techniques allow to study interesting system properties without requiring the state space generation. In order to avoid the net unfolding, which would reduce significantly the effectiveness of the analysis, a symbolic calculus allowing to directly work on the WN colour structure is needed. The algorithms for high level Petri nets structural analysis most often require a common subset of operators on symbols annotating the net elements, in particular the arc functions. These operators are the function difference, the function transpose and the function composition. This paper focuses on the first two, it introduces a language to denote structural relations in WN and proves that it is actually closed under the difference and transpose.