The sequence construct is a list of activities, separated by
a semi-colon, such that each subsequent activity is only performed after the
completion of the preceding activity.

"{"
    ( <Activity> ";" )*
"}"

where Activity<i> represents any protocol based activity
or construct.

The following example shows a sequence of interactions.

{
    Order from Buyer to Seller;
    Invoice from Seller to Buyer;
    Payment from Buyer to Seller;
    Confirmation from Seller to Buyer;
}