x CASE y OF A ENDOF z OF B ENDOF C ENDCASE: If x == y, execute
A, if x == z, execute B. Otherwise, execute C. x is dropped
in case of an OF match, *but it is kept if it reaches C*. You
have to consume it to avoid PSP leak.           

(br)      --   Branches by the number specified in the 2
               following bytes. Can be negative.
(?br)     f -- Branch if f is false.
(         --   *I* Comment. Ignore input until ")" is read.
[         --   Begin interpretative mode. In a definition,
               execute words instead of compiling them.
]         --   End interpretative mode.
ABORT     --   Resets PS and RS and returns to interpreter.
ABORT" x" --   *I* Compiles a ." followed by a ABORT.
ERR       a -- Prints a and ABORT. Defined early and used by
               drivers.                                 (cont.)