#\STOP-OUTPUT in Maclisp

[kmp,rwk at MIT-MC]

We've decided that #\STOP-OUTPUT and friends should do something like a 
CONS-A-READ-UNDEFINED-OBJECT so that the object read in is
#<READ-UNDEFINED-OBJECT "#\STOP-OUTPUT">
or something to that effect. That way, the compiler can detect the error
reliably, and the interpreter will stand a fair chance of providing a useful
diagnostic, too. Eg, EVAL'ing #<READ-UNDEFINED-OBJECT ...> should presumably
err immediately.

If this really bothers someone please let us know. It probably won't go in
immediately. 

-kmp