The structures notation (see section 5.1) is used to define and access variable attributes and their arguments. This makes the code independent of the number of attributes and positions of their arguments. Wherever appropriate, the libraries described in this document describe their attributes in this way, e.g.,
suspend{inst:I, constrained:C, bound:B}
says that the structure name is suspend and that it has (at least) three arguments with the corresponding names.