5 Nested invocation

Most programs have subroutines calling other subroutines. There is nothing mysterious about nested invocations. Just follow the same steps as a single invocation.

Algorithm 3 is an example containing nested invocataions.

The trace of this algorithm is in table 3.

Table 3: A trace of algorithm 3.

line #			comments
8			first line out of a subroutine
9	abc		subroutine ``abc'' stores the line # to return to
	return line #
	10
2			the first line in the subroutine to execute
3		def	we invoke ``def'' from ``abc''
		return line #	this is the return line # that belongs to ``def'', it is independent to the return line # of ``abc''
		4	this is the line immediately following the invocation
6			this is the only line to execute in ``def''
7		return	now we return from ``def'' to ``abc'', note which column we use for the return line #, this ``return line #'' column is destroyed after this row in the trace
4	return		we are ready to return from ``abc'', use the stored return line #. Note, again, which column we use to return
10			now we are back to the ``main program''