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Let P be the plane defined by the points p, q,
and r. Note that the order defines the orientation of
P. The function computes the orientation of points p,
q, and s in P: Iff p, q, s are
collinear, COLLINEAR is returned. Iff P and the plane
defined by p, q, and s have the same orientation,
POSITIVE is returned; otherwise NEGATIVE is returned.
| Precondition: | p, q, r, and s are coplanar and
p, q, and r are not collinear. |
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If p,q,r are collinear, then COLLINEAR is returned.
If not, then p,q,r define a plane P. The return value in this case is
either POSITIVE or NEGATIVE, but we don't specify it explicitly.
However, we guarantee that all calls to this predicate over 3 points in P
will return a coherent orientation if considered a 2D orientation in P.
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