collapseos/blk/428

( At this point, we have something like "0 e1 e2 e3 val". We
  want top drop val, and then call THEN as long as we don't
  hit 0. )
: ENDCASE
    BEGIN
        DUP NOT IF
            DROP COMPILE R> COMPILE DROP EXIT
        THEN
        [COMPILE] THEN
    AGAIN
; IMMEDIATE