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//
// Conventions:
//
// +x = left
// +z = rear
// +y = up
// +pitch = nose up
// +yaw = nose right
// +roll = right
//
// [BODY] - a rigid mass with mass and inertial properties
// [JOINT] - a ball joint constraining an offset of one body to an
// offset of another body (eliminates 3 DOF)
// [HINGE] - a constraint restricting the relative rotations of two
// bodies to be around a single axis (eliminates 2 DOF).
// [BAR] - a constraint holding an offset of one body from an offset of
// another body at a fixed distance (eliminates 1 DOF).
// [JOINT&HINGE] - both the joint and hinge constraints, forming the
// conventional definition of a hinge (eliminates 5 DOF).
//
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