### Quadratic number fields

Hello!

I try to implement an inclusion test, of which I thought it must be quite simple, but until now I cannot figure out how to do it properly.
Problem: Given any element x, test if x is included in some quadratic number field and return true/false.
I tried the following:

x = 2 + sqrt(5)
K = QuadraticField(5)
print(x in K)

Here, the answer is 'no' or 'false' which is obviously wrong, so I assume I'm doing something completely wrong. What do I have to change?