``` 100 IF n=0 THEN PRINT z$: ENDPROC 120 IF n=1 THEN LET l=LEN z$ 130 a$=z$(l)+z$+z$(l) 140 PRINT z$' INVERSE 1; a$ 150 ENDPROC 160 STOP ``` As you can see, there's a default value for parameter n and two separate exit points for procedure **HelloWorld**: one at line 100 and one at line 150. Line 40 is mandatory or rather a condition to jump over the procedures defined is mandatory as without it, after execution of both procedures the next available line would have been 50. **DEFPROC** can only appear in a program line. Attempting to define a procedure interactively will result in the error **Direct Command Error**. Also, supplying the wrong type of variable as a parameter (ie. a string instead of a number) will result in the error: **Q Parameter error**. Executing the program will return the following:  ``` 11 risen to the 2nd power is :12 1 Hello World! !Hello World!! Hello Stop! 9 STOP statement, 160:1 ``` *Fig. 3 - Screen output from the example procedures* As we saw in the definition of the **DEFPROC**, **ENDPROC** and **PROC** keywords as well as the examples above, there are optional parameters that can be passed to procedures when called with the results of the procedures' execution being assigned to multiple variables at the time. Consider this example that calculates the factorial of a number: ``` 10 INPUT "Enter a number 1+:";x 20 IF x>33 THEN PRINT "Your Next cannot handle this number!": GO TO 999 30 PROC factorial(x) TO f 40 IF f>0 THEN PRINT "The factorial of ";x;" is ";f: ELSE GO TO 999 999 STOP 1000 DEFPROC factorial(n) ```