Function: qfbprimeform
Section: number_theoretical
C-Name: primeform
Prototype: GG
Help: qfbprimeform(x,p): returns the prime form of discriminant x, whose
 first coefficient is p.
Doc: prime binary quadratic form of discriminant
 $x$ whose first coefficient is $p$, where $|p|$ is a prime number.
 By abuse of notation,
 $p = \pm 1$ is also valid and returns the unit form. Returns an
 error if $x$ is not a quadratic residue mod $p$, or if $x < 0$ and $p < 0$.
 (Negative definite \typ{QFB} are not implemented.)
