question: public static S2Cap fromAxisAngle ( S2Point axis , S1Angle angle ) { double d = Math . sin ( 0.5 * angle . radians ( ) ) ; return new S2Cap ( axis , 2 * d * d ) ; }
target:  Create a cap given its axis and the cap opening angle, i.e. maximum angle between the axis and a point on the cap. 'axis' should be a unit-length vector, and 'angle' should be between 0 and 180 degrees.