HP 35s Scientific Calculator - Manual (Page 186)

13-4 Simple Programming
Program Boundaries (LBL and RTN)
If you want more than one program stored in program memory, then a program
needs a label to mark its beginning (such as   ) and a return to mark its
end (such as  ).
Notice that the line numbers acquire an  to match their label.
Program Labels
Programs and segments of programs (called routines) should start with a label. To
record a label, press:
  letter–key
The label is a single letter from A through Z. The letter keys are used as they are for
variables (as discussed in chapter 3). You cannot assign the same label more than
once (this causes the message ), but a label can use the same
letter that a variable uses.
It is possible to have one program (the top one) in memory without any label.
However, adjacent programs need a label between them to keep them distinct.
Programs can not have more than 999 lines.
Program Returns
Programs and subroutines should end with a return instruction. The keystrokes are:
 
When a program finishes running, the last RTN instruction returns the program
pointer to  , the top of program memory.
Using RPN, ALG and Equations in Programs
You can calculate in programs the same ways you calculate on the keyboard: