Tricks in BASIC

[GCHarder]

Here's a "Bulletproof" version, no B/XX errors. At least I couldn't break it. It is a bit larger though.

30 LET I$=INKEY$
40 IF I$<"A" OR I$>"Z" THEN GOTO 30
50 LET I=VAL "00700000000005016432000000"(CODE I$-37)
60 IF NOT I THEN GOTO 30
70 GOTO 100*I

Run size, 1413