SPLEX “Special BASIC file utilities”.  SIZE: 1250 bytes.

• LIST$ returns any program line of any BASIC program in memory, in the form of a
string that is identical to the way the program line would be LISTed. This allows
a program to examine the contents of another BASIC program, or even itself,
without needing to TRANSFORM it into text.

• MODIFY takes a string (a valid line of program BASIC) and merges it into the
specified program. This allows a BASIC program to modify any other BASIC program,
or even itself, without POKE.

• Together, LIST$ and MODIFY allow such things as a telephone directory program
with all its data in DATA statements that are updated by the program itself.

Syntax:
LIST$(line number, optional filename)
MODIFY filename, new line as a string including the line number

If LIST$ is not given a filename, the currently running program is used.
The string returned by LIST$ does NOT contain the line number.
If you wish to MODIFY the currently running program, use CAT$(0) as the filename.
You don't have to specify CAT$(0)[1,10].

*** SPLEX ***
(31D40) File Header
(31DAF) Poll Handler
(31E0D) 52/24 LIST$(#,?$)
(31FB9) 52/23 MODIFY filename$, newline$
(325FC) Poll Handler
(32638) 53/55 MAXRC fn
(32729) Next File Header

Note: The MAXRC command is only used by SPREAD, a commercial spreadsheet program
by Chris Bunsen, who also wrote SPLEX.

Examples of SPLEX commands in use:

U$=LIST$(ERRL)
-- line that errored in the current program

MODIFY CAT$(0),STR$(ERRL)&" "&U$
-- modify current program's error line to contain U$

0050 IF LEN(LIST$(M+10)) THEN M=M+10 @ GOTO 50
-- exits with M = next non-existent line numbered by tens

Bad bug in SPLEX:
Type 10 DIM A(MEM/16).
Then type A$=LIST$(10).
You will immediately see Memory Lost!
The problem is the MEM in the DIMension.
In general, any lexfile function (including LEX 01, the built-in lex file)
used in a DIM statement of any kind (like INTEGER, COMPLEX, etc.)
when pulled out by the LIST$ function during an assignment to a string variable,
causes Memory Lost.
So anything like A$=LIST$(10,F$) when F$'s line 10 is INTEGER M(MAXRC)
will always wipe out.