Semantics
The segment defined between the ?<begin> ?<end> operators will be replaced by “sub expression 2” when “sub expression 1” evaluates to true, and by “sub expression 3” when “sub expression 1” evaluates to false.
Take another look at example 1:
The API line will be expression when yes if
$<prop1>
evaluates to prop1val and will be expression when no otherwise.