The proper way to insert lines of code between two existing lines is to use line number within the interval of the two line numbers.Renum command may be used to renumber the program lines after those modifications.
Despite nuBasic interpreter editor uses line numbers, you don't need them in your programs.
The interpreter does not need line numbers to handle program flow if you use labels with GoTo/GoSubstatements.
If your program does not refer line numbers (but just labels), nuBASIC strips line numbers during file saving and reinsert them automatically when program is loaded at later time.
In this case, line numbers are provided only for editing and debugging purpose.
he following program is considered 'old-style' because of using of numbered reference in a GoTo statement (GoTo statement at 40 points to line 20).
This old style program maintains line numbers when you save it in a file:
Using command Meta you may verify if a program has explicit line number references.
Line number reference with GoTo or GoSub statements is deprecated.