NAME:
  fn_hg3

 PURPOSE:
  Evaluate the 3-parameter Henyey-Greenstein phase function.

 DESCRIPTION:

 CATEGORY:
  Mathematical

 CALLING SEQUENCE:
  fn_hg3, g, a, F, /radians

 INPUTS:
     g: Phase angle(s) in degrees.
     a: Array of parameters:
         A[0] = forward asymmetry (should be positive)
         A[1] = backward asymmetry (should be negative)
         A[2] = forward fraction (between 0 and 1)

 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:
     radians:  Phase angle is in radians instead of default degrees.

 OUTPUTS:
     F: The value of the function at each g.

 KEYWORD OUTPUT PARAMETERS:

 OPTIONAL OUTPUT PARAMETERS:

 COMMON BLOCKS:
  None.

 SIDE EFFECTS:
  None.

 RESTRICTIONS:
  None.

 PROCEDURE:
 MODIFICATION HISTORY:
  1999-Nov-13 Created by Will Grundy
  2001-Oct-29 WMG added radians keyword