Function: Qfb
Section: conversions
C-Name: Qfb0
Prototype: GDGDG
Help: Qfb(a,{b},{c}): binary quadratic form a*x^2+b*x*y+c*y^2.
Doc: creates the binary quadratic form\sidx{binary quadratic form}
 $ax^{2}+bxy+cy^{2}$. Negative definite forms are not implemented,
 use their positive definite counterpart instead.
 The syntax \kbd{Qfb(V)} is also allowed with $V$ being either
 a \typ{VEC} $[a,b,c]$, a \typ{POL} $ax^{2}+bx+c$ or a \typ{MAT}
 $[a,b_{0};b_{1},c]$ with $b_{0}+b_{1}=b$.
