Slide 40 of 41
Notes:
Let’s just go back briefly to PER.
PER never encodes tags, but the rules on when tags have to be distinct still applies to legal use of the notation, and PER exploits this by using tags to determine a fixed order for alternatives of choices and elements of sets that is independent of the textual order.
In the case of choice, the order determines the value of the so-called choice index that is encoded at the head of each choice to identify the chosen alternative. In the case of set, the order determines the order in which an encoder is required to encode the elements (and hence tells a decoder which encoding belongs to which element).
