We denote a sequence of identifiers with 
 and 
. As a convention, we use 
 for constants 
 and 
 for variables of an abstract or a concrete machine respectively 
 and 
 for parameters of an abstract or concrete machine respectively 
 for axioms 
 and 
 for the invariants of the abstract machines or concrete machine respectively 
 and 
 for the guards of the abstract events or concrete event respectively