for self-modification to work, THEN is mandatory before line numbers

README.md

@@ -12,7 +12,7 @@ The LTTB specification expands upon the original Design Notes in the following m
 - Any invalid statement (including `REM`) can be used for comments.
 - No command can be abbreviated inside `IF` statements.
 - `LET` and `THEN` keywords are optional.
-- `IF` statements also allow inserting line numbers after `THEN`, making it possible to write self-modifying programs.
+- `IF` statements also allow inserting line numbers after `THEN`, making it possible to write self-modifying programs. In this case, `THEN` is mandatory.
 - A special `RND(n)` function is introduced that generates a random number from 0 to `n-1`.
 - The `NEW` command is synonym to `CLEAR`.
 - The `BYE` command can be used to exit the interpreter.

lttb-manual.md

@@ -62,7 +62,7 @@ Execute a statement `[stmt]` if the relation denoted by `relop` holds between `[
 
 Examples: `IF A > B THEN LET C = A-B`, `IF X<1000 THEN GOTO 400`
 
-Non-standard extension: if the `[stmt]` statement starts with a line number, LTTB doesn't execute it immediately but instead overwrites the original line with the number it starts with. This allows writing self-modifying programs in LTTB.
+Non-standard extension: if `THEN` is specified **explicitly** and the `[stmt]` statement starts with a line number, LTTB doesn't execute it immediately but instead overwrites the original line with the number it starts with. This allows writing self-modifying programs in LTTB.
 
 ### `GOTO [expr]`