deep-equal(A, B) means (count(A) eq count(B) and every $i in 1 to count(A)
satisfies A[$i] eq B[$i]). Therefore deep-equal((), ()) is true.

Is it fair to say deep-equal could mean:

every $i in 1 to max((count(A), count(B))) satisfies A[$i] eq B[$i]

which would then return false, as its only the length check that is
causing it to return true (and thats only there to avoid also checking
every item in B is equal to A)

Michael Kay
Saxonica