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## astAngle - Calculate the angle subtended by two points at a third point

Description:
This function finds the angle at point B between the line joining points A and B, and the line joining points C and B. These lines will in fact be geodesic curves appropriate to the Frame in use. For instance, in SkyFrame, they will be great circles.
Synopsis:
double astAngle( AstFrame this, const double a[], const double b[], const double c[] )
Parameters:
this
Pointer to the Frame.
a
An array of double, with one element for each Frame axis (Naxes attribute) containing the coordinates of the first point.
b
An array of double, with one element for each Frame axis (Naxes attribute) containing the coordinates of the second point.
c
An array of double, with one element for each Frame axis (Naxes attribute) containing the coordinates of the third point.
Returned Value:
astAngle
The angle in radians, from the line AB to the line CB. If the Frame is 2-dimensional, it will be in the range \$pm pi\$, and positive rotation is in the same sense as rotation from the positive direction of axis 2 to the positive direction of axis 1. If the Frame has more than 2 axes, a positive value will always be returned in the range zero to \$pi\$.
Notes:
• A value of AST__BAD will also be returned if points A and B are co-incident, or if points B and C are co-incident.

• A value of AST__BAD will also be returned if this function is invoked with the AST error status set, or if it should fail for any reason.

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AST A Library for Handling World Coordinate Systems in Astronomy