#radian__rad  the unit of plane angle adopted under the System International d'Unites; equal to the angle at the center of a circle subtended by an arc equal in length to the radius (approximately 57.295 degrees)
  supertype:  #angular_unit__angularunit  a unit of measurement for angles
  part:  #milliradian  a unit of angular distance equal to one thousandth of a radian
     part:  #microradian  a unit of angular distance equal to one thousandth of a milliradian

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