HP 35s User Manual page 213

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
a b c d
2 2 2 2
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.
14-3
Programming Techniques