Function: Qfb
Section: conversions
C-Name: Qfb0
Prototype: GGGDGp
Help: Qfb(a,b,c,{D=0.}): binary quadratic form a*x^2+b*x*y+c*y^2. D is
 optional (0.0 by default) and initializes Shanks's distance if b^2-4*a*c>0

