bug in equation checking for quasi projective/affine schemes

[davideklund]

The method _check_satisfies_equations of the class AlgebraicScheme_quasi checks whether a point p lies on the complement of a scheme Y in a scheme X.

If one of the equations defining Y vanishes, then p is judged not a point on X-Y, but it could be that some other equation defining Y does not vanish.

Example:

sage: P.<x, y, z, w> = ProjectiveSpace(3, QQ)
sage: S = P.subscheme([x])
sage: T = P.subscheme([y, z])
sage: U = T.complement(S)
sage: U._check_satisfies_equations([0,0,1,1])
...
TypeError: Coordinates [0, 0, 1, 1] do not define a point on Quasi-projective subscheme X - Y of Projective Space of dimension 3 over Rational Field, where X is defined by:
  x
and Y is defined by:
  y,
  z

Apply trac_12211_fix_bug.2.patch​

[davideklund]

Fixes the bug.

[vbraun]

Can you add your example to the doctest?

[davideklund]

Fix bug, contains doctest (patch made with Sage 4.7.2).

[vbraun]

Looks good!