is mentioned by

Def a n == if 0aa n ;((-a) rem n)=0-((-a) n) else -((-a) n)+-1 fi	[div_floor]
Def a mod n == if 0aa rem n ;((-a) rem n)=00 else n-((-a) rem n) fi	[modulus]

In prior sections: bool 1

Try larger context: StandardLIB