NAME:
	hgp
 PURPOSE: (one line)
	Compute a single parameter Henyey-Greenstein phase function.
 DESCRIPTION:
 CATEGORY:
	Mathematical
 CALLING SEQUENCE:
	ans = hgp(phase,c)
 INPUTS:
	phase - Angle between the incident and emitted flux (in degrees).
	c     - Henyey-Greenstein functional value.
 OPTIONAL INPUT PARAMETERS:
 KEYWORD PARAMETERS:
 OUTPUTS:
	Return value is the phase function value(s).  Same dimension as
	the inputs.
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
	Written 1991 Feb.,by Marc W. Buie, Lowell Observatory