SOURCELINE

returns the line number of the final line in the program if you omit n or 0 if the implementation does not allow access to the source lines. If you specify n, returns the nth line in the program if available at the time of execution; otherwise, returns the null string. If specified, n must be a positive whole number and must not exceed the number that a call to SOURCELINE with no arguments returns.

SOURCELINE()    ->   10
SOURCELINE(1)   ->   '/* This is a 10-line REXX program */'