Actions indicate how to change the data structure: for instance, "`Add 42" to add the int 42 to a set data structure.
Type of functions to be called each time the data structure has been updated.
Type of functions to be called each time the data structure has been updated and the data structure containing it (if any) (nested data structures) has to be updated.
Unique identifiers for sources; allows checking whether a message has the same emitter as a previous one.
The type of the data structure.