This has its own operation system, which is translated into MC commands when the filter is run. According to the documentation:
The other important thing is that as of now you are not allowed to use the scoreboard players operation command in RPL, because it will confuse the operation signs with its own format. RPL implements all the features of this command, so you probably won't need to use it anyway.
The documentation describes this other system, as well as many other systems to make your code cleaner and easier to write. If you do need to use operation, place a $ as the first character on the line so RPL doesn't try to translate it.
It would be useful if the length of each line were made available as relative coords at compile-time. Then it would be possible to implement recursion by having a copy of a line as each stack frame, stored at an offset corresponding to its return address, and growing the stack perpendicular to the instruction flow. (The offset would thus serve as a return address, and we wouldn't have to worry about having parallel processes collide.) For instance, if all lines ran in +x direction, and the stack was embedded in bedrock to save relighting cycles, then gosub would be accomplished by