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   Computes an RF torque for a set of antenna.
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   Form:
   t = TRF( u, r, p )
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   Inputs
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   u                  (3,n or 1) Antenna unit vectors
   r                  (3,n or 1) Moment arms (m)
   p                  (1,1)      Power (watts)

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   Outputs
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   t                  (3,1)      Torque (Nm)

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