It is possible to build a number system native to Modal.

The language does not accommodate for any specific numerical system, but allows for the notion of numbers to be implemented with Peano Numerals:

<> (add (s ?x) (s ?y)) (s (add ?x (s ?y)))
<> (add (s ?x) (0)) (s ?x)
<> (add (0) (s ?y)) (s ?y)
<> (add (0) (0)) (0)
<> (sub (s ?x) (s ?y)) (sub ?x ?y)
<> (sub (s ?x) (0)) (s ?x)
<> (sub (0) (s ?y)) (s ?y)
<> (sub (0) (0)) (0)
<> (mul (s ?x) (s ?y)) (add (s ?x) (mul (s ?x) (sub (s ?y) (s (0)))))
<> (mul (s ?x) (s (0))) (s ?x)
<> (mul (s (0)) (s ?y)) (s ?y)
<> (mul (s ?x) (0)) (0)
<> (mul (0) (s ?x)) (0)

To convert from prefix notation to infix:

<> (?x + ?y) (add ?x ?y)
<> (?x - ?y) (sub ?x ?y)
<> (?x * ?y) (mul ?x ?y)

Altogether, we have enough parts to implement factorial:

<> (factorial (s (0))) ((s (0)))
<> (factorial (s ?x)) (((s ?x) * factorial ((s ?x) - (s (0)))))

factorial (s (s (s (s (s (0))))))

Binary

Prefix rules to increment a binary number:

<> (inc (0 ?x)) ((1 ?x))
<> (inc (1 ?x)) ((0 inc ?x))
<> (inc ()) ((1 ()))

?(?-) (Count to 0x7f)

<> (> increc (1 (1 (1 (1 ()))))) (done.)
<> (> increc ?i) (> (inc ?i wait) increc)
<> (> (?i wait) increc) (> increc ?i)

> increc ()