HP 35s Scientific Calculator - Manual (Page 213)

Programming Techniques 14-3
MAIN program
(Top level)
End of program
Attempting to execute a subroutine nested more than 20 levels deep causes an 
 error.
Example: A Nested Subroutine.
The following subroutine, labeled S, calculates the value of the expression
as part of a larger calculation in a larger program. The subroutine calls upon
another subroutine (a nested subroutine), labeled Q, to do the repetitive squaring
and addition. This saves memory by keeping the program shorter than it would be
without the subroutine.
2 2 2 2 d c b a
+ + +