Let's ask the interpreter / compiler:

  ./xl -nobuiltins -parse /tmp/glop.xl -style debug -show
  (infix is
   is
   is
  )

So what it sees is a definition of a name 'is' as itself.

Now, that name is very unlikely to be usable because `is` as an infix is so central to everything. As a matter of fact, it looks like even "is is 2" actually crashes the current implementation.

Oh well. I wonder what a sensible error message on this would be. Probably: "Bill Clinton denied to comment on the meaning of that statement".