A little m4 puzzle

Consider this predicate on strings.

   define(eq,`define(`_$1',F)define(`_2',T)_$1')

eq(x,y) yields T or F according as strings x and y are
the same or different. (The strings may contain only
characters allowed in macro names).

The puzzle: If you omit inner quotes from the definition,
eq(A,A) yields T as intended. But what happens when you
repeat the test?

(Incidentally, you can replace T by `$3' and F by `$4'
to make a near replacement for the builtin ifelse.)