ASSERT-FIXNUM

[RWK at MIT-MC (Robert W. Kerns)]

Actually, in compiled code it will blindly assume it's a fixnum and treat
it like one, without checking to see if the assumption is valid.  In the
interpreter, it merely checks at that time, it doesn't assert it in any
database or anything.  So ASSUME is the more appropriate term here, I would
assert.  I assume you'll agree.