// compute intersection of line passing through a and b
// with line passing through c and d, assuming that unique
// intersection exists; for segment intersection, check if
// segments intersect first
// **use LinesParallel and LinesColliner to detect wether they intersect
point ComputeLineIntersection(point a, point b, point c, point d)
{
b = b - a ;
d = c - d ;
c = c - a ;
assert( b.dot(b) > EPS && d.dot(d) > EPS);
return a + b* c.cross(d) / b.cross(d) ;
}
bool observable(point p, point q, double a) {
return acos(p.dot(q)/p.abs()/q.abs()) < a+eps;
}
friend double dot(const point & p, const point & q) { return p.dot(q); }
