### strange way to simplify square roots

Hi!

Let us consider the number

a=1/sqrt(4-2*sqrt(3))-1/(sqrt(3)-1)

which is zero. If we simplify

a.simplify_full().n()

we obtain

-2.73205080756888 - 8.36449319149292e-17*I,

which of course is false. I guess it is a problem handling complex roots of degree four, but still, it don't seems to me a reasonable computation. ¿Anybody knows how to fix it?