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Point A - the position associated with vector R.
Point B - the position associated with vector T.
Vector AB - the vector from point A to point B
Angle beta - the angle between vector R and vector AB.
Angle theta - the angle between vector T and vector AB
theta = asin( |R| * sin(beta) / |T| )
beta = acos (AB.xR.x + AB.yR.y)
, asin acos -PI/2 PI/2.
beta = atan2( R.y, R.x ) - atan2( AB.y, AB.x )
x = |R| * sin(beta) / |T|
y = 1 + sqrt( 1 - x*x )
theta = 2*atan2( y, x )
, x > 1 R
..
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