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   Converts orbital elements to commonly used angular quantities
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   Form:
   [L, K, l] = El2LKl( el )
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   ------
   Inputs
   ------
   el                    Elements vector [a,i,W,w,e,M]

   -------
   Outputs
   -------
   L                     True longitude      ( w + W + f )
   K                     Eccentric longitude ( w + W + E )
   l                     Mean longitude      ( w + W + M )

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