Function: quadgen
Section: number_theoretical
C-Name: quadgen
Prototype: G
Help: quadgen(D): standard generator of quadratic order of discriminant D.
Doc: creates the quadratic
 number\sidx{omega} $\omega=(a+\sqrt{D})/2$ where $a=0$ if $D\equiv0\mod4$,
 $a=1$ if $D\equiv1\mod4$, so that $(1,\omega)$ is an integral basis for the
 quadratic order of discriminant $D$. $D$ must be an integer congruent to 0 or
 1 modulo 4, which is not a square.