Subroutine invocation and returning

When a subroutine is invoked, two operations occur:

First, the left-most un-allocated column is allocated and labeled as a “return line number”, or abbreviated to “retline#”. This column then gets a value that is the number of the line that logically follows the invocation. The purpose of this column is to remember where to continue execution when the subroutine is finished.
Second, execution continues on the first action line in the invoked subroutine.

When execution reaches the end of the a subroutine, marked “end define sub” in pseudocode, the following two operations occur:

First, the right-most allocated column labeled “retline#” is looked up. This column indicates where to continue execution. Write the number in the next row of the trace (in the “line#” column).
Second, the “retline#” that was just utilized is marked deallocated. Do so by using a series of “XXXXXXX” characters where the column is deallocated. To be more clear, you can also use a strike-out font, such as “retline#” to indicate the column is deallocated.