Fix Sage's Brill-Noether

See the files

• SAGE_ROOT/devel/sage/sage/schemes/plane_curves/projective_curve.py
• SAGE_ROOT/devel/sage/sage/schemes/plane_curves/affine_curve.py

in the Sage install. Note this in the funciton riemann_roch_space (in projective_curve.py):

        The following example illustrates that the Riemann-Roch space
        function in Singular doesn't *not* work correctly.
        
        ::
        
            sage: R.<x,y,z> = GF(5)[]
            sage: f = x^7 + y^7 + z^7
            sage: C = Curve(f); pts = C.rational_points()
            sage: D = C.divisor([ (3, pts[0]), (-1,pts[1]), (10, pts[5]) ])
            sage: C.riemann_roch_basis(D)    # output is random (!!!!)
            [x/(y + x), (z + y)/(y + x)]
        
        The answer has dimension 2 (confirmed via Magma). But it varies
        between 1 and quite large with Singular.

• Sage's trac 8997.

• Singular's trac 153.

There is a wiki page about general function fields issues in Sage.