A macrocode level dispatch instruction.

 PUSH N
 PUSH I
 %NCASE
PC=>

* If I is of wrong type or not  0<=I<N then use N.
 POP N, POP I, PC = PC+I.




(SELECTQ X
 (A (FOO))
 (B (BAR))
 (T (BAZ)))

==> PUSH 2
    (CDR (ASSQ X '((A . 0) (B . 1))))
    %NCASE
    GOTO FOO-CASE
    GOTO BAR-CASE
BAZ-CASE:
  ....