In http://www.numbertheory.org/php/squareroot.html the case "p divides a" has two subcases,
``p**n divides a`` and ``gcd(a, p**n) = p**r, 0 < r < n``, as in ``_sqrt_mod1``, with the same notation;
in the second subcase one has ``r = factorint(a)[p]``.