A block header may define one or more capture variables. These can be used on certain keyword calls to provide one or more symbols that shall reference relevant values. Capture variables are not assignable.
The \if keyword allows a capture variable in its then argument if the given condition is an \Optional expression.
The variable can then be used to reference the existing value.
The \for keyword allows for up to two capture variables in its body, with the first referencing the current item, and the second referencing its index.