% asir
[0] load("fac")$
[3] debug$
(debug) list factorial
1   def factorial(X) {
2       if ( !X )
3           return 1;
4       else 
5           return X * factorial(X - 1);
6   }
7   end$
(debug) stop at 5                     <-- setting a break point
(0) stop at "./fac":5
(debug) quit                          <-- leaving the debug-mode
[4] factorial(6);                     <-- call for factorial(6)
stopped in factorial at line 5 in file "./fac"
5           return X * factorial(X - 1);
(debug) where                         <-- display the calling sequence
factorial(), line 5 in "./fac"            up to this break point
(debug) print X                       <-- Display the value of X
X = 6 
(debug) step                          <-- step execution
                                          (enters function)
stopped in factorial at line 2 in file "./fac"
2       if ( !X )
(debug) where
factorial(), line 2 in "./fac"
factorial(), line 5 in "./fac"
(debug) print X
X = 5 
(debug) delete 0                      <-- delete the break point 0
(debug) cont                          <-- continue execution
720                                   <-- result = 6!
[5] quit;