incrementCurrentLine method Null safety
Increment the current line, up to a max of lines, wrapping to 0. To be clear, there are lines+1 possible values.
Note that the branching instructions can cause the program to increment past the phantom RTN instruction at the end of memory, wrapping back to line 1. This is intentional, and mirrors the behavior I observed on my 15C.
void incrementCurrentLine() => currentLine = (currentLine + 1) % (lines + 1);