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      Convert from the Grating diffracted Angular coordinates (GAC) to Grating Dispersion Plane (GDP) coordinate system.
     
Array_Type pix_gac_to_gdp( Double_Type x, Double_Type y ) 
      
	This routine converts a position in the Grating diffracted
	Angular Coordinate (GAC) system
	to the matching position in the Grating Dispersion Plane (GDP) coordinate system,
	using the current settings of the pixlib module.
	The x value gives the grating dispersion and
	the y value the cross-dispersion coordinate, both given
	in degrees.
        The return value is a two-element array which gives the
        GDP coordinates in pixels.  
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