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      Return the photon energy corresponding to a given grating dispersion (GAC) value.
     
Double_Type pix_get_energy( Double_Type x, Double_Type y ) 
      
	Calculates the photon energy, in keV, for a given grating
	position and using the current pixlib settings.
	The x and y values are in the GAC coordinate system, and so
	represent the grating dispersion and cross-dispersion
	angles in degrees. The grating order - as set by
	pix_set_grating() - must be non-zero for this routine
	to work.
       
	The pix_get_grating_wavelength() routine can be used to find
	the wavelength of the photon instead of the energy.
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