Exact GCD of integer polynomials

The polynomial GCD of two given polynomials can be found exactly if the polynomial coefficients are all integers.

The process involves only integer arithematic operations. The floating point errors may be completely eliminated.

The expected result is obtained by applying recurrently
"Elemination of leading coefficients of two equal-degree
polynomials." It is modified from "Monic polynomials subtraction," presented previously.

One of the main application is to factorize a integer polynomial with multiple roots.