System Function STOP
From NARS2000
Quad Stop  ⎕STOP (System Function)
Monadic Function — Query STOP Property


Z is a numeric vector of line numbers where the function named in R stops execution when the corresponding line number is encountered. A value of 0 in the result means that execution stops just prior to leaving the function, allowing the user to examine the result and all local variables. 
For example:
⎕STOP 'f'
0 1 2 3
⎕STOP '#DydDotDot'
0 2
Dyadic Function — Set STOP Property


R is a character scalar or vector name of a userdefined function/operator, anonymous function, or Magic Function.  
L is a numeric vector of line numbers in the named function where execution is to stop when the corresponding line number is encountered. If L is an empty vector, the STOP property of the named function is removed from all line numbers. If a number in L does not correspond to a line number in the named function, that number is ignored.  
Z is a numeric vector subset of L where the named function stops execution when the corresponding line number is encountered. The values in the result are in ascending order. A value of 0 in the result means that execution stops just prior to leaving the function, allowing the user to examine the result (if any) and all local variables. The Display property of the result is OFF, meaning that the result does not display unless explicitly forced such as with ⎕←. 
For example:
⎕←0 1 2 3 ⎕STOP 'f'
0 1 2 3
⎕←0 2 3 4 5 6 7 8 9 ⎕STOP '#DydDotDot'
0 1 2 3 4 5