NAME: 
  disphase
 PURPOSE: 
  Apply distance and phase angle correction to observed magnitudes.
 DESCRIPTION:
  Apply IAU standard asteroidal-law corrections to observed magnitudes
     given the distance, phase angle, and the G coefficient.  Magnitudes
     are corrected to 1 AU from Sun and Earth and to 0 degrees phase angle.

  Note that the computation blows up in single precision if G < -0.85.  If
     the input value for G is out of range then the returned magnitude is
     set to -99.99.  This prevents getting back a value of NaN for hmag.
 CATEGORY:
  Photometry
 CALLING SEQUENCE:
  disphase,mag,r,d,phang,g,hmag
 INPUTS:
     mag   - Observed magnitude.
     r     - Sun-object distance in AU.
     d     - Earth-object distance in AU.
     phang - Phase angle of observation in degrees.
     g     - IAU standard G value (phase angle coefficient).
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
 OUTPUTS:
     hmag  - Magnitude corrected for distance and phase angle.
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
   2008/04/12, MWB, slight change to deal with bogus values for G