It may contain procedures. It is intended to be attached to any other program unit where the entities defined within it become accessible.
Numbers are growing, many are free.
It is also possible to declare the function type in the declarations area instead of in the header. Numberset is an actual argument and is the argument associated with the dummy argument Numbers. For the above call, in OutputFigures, the name Numbers is an alias for NumberSet. The actual arguments a and b are associated with the dummy arguments x and y. If the value of a dummy argument changes then so does the value of the actual argument.