`modular-inverse``a``n`procedure- a
- integer
- b
- integer

Returns the inverse of a modulo

`n`if`a`and`n`are coprime, otherwise raises an error. The modulus`n`must be positive, and`a`must be nonzero.Formally, if

`a`and`n`are coprime,`b`=`(modular-inverse a n)`is the unique natural number less than`n`such that`(* a b)`=`1`(mod`n`).> (modular-inverse 2 5) 3 > (modulo (* 2 3) 5) 1

Wikipedia: Multiplicative Inverse