function eit_gt_corner, header
;+
; Name:
;    eit_gt_corner
;
; Purpose:
;    Return the defining corners of an EIT image.
;
; Input Parameters:
;    header = FITS header or EIT index structure for image
;
; Output:
;    corners = [P1_X, P2_X, P1_Y, P2_Y]
;
; Calling Sequence:
;    corners = eit_gt_corner(index)
;    corners = eit_gt_corner(fits_header)
; 
; Restrictions:
;    The one defining reason for this routine is to work around the
;    COMMENT header definitions.  If P[12]_[XY] ever were to become 
;    real FITS header keywords instead of comments, it is my hope that
;    we would only have to fix this one spot to deal with it.
;
; History:
;    31-October-96 - (BNH) - Written
;-
if (n_elements(header) eq 0) then begin
   message, /info, 'No index/fits header supplied!'
   return, -1
endif

if ((data_chk(header,/type) ne 7) and (data_chk(header,/type) ne 8)) then begin
   message, /info, 'Argument is not a valid fits header/index structure.'
   return, -1
endif
 
a = intarr(4)
a(0) = eit_fxpar(header, 'P1_X')
a(1) = eit_fxpar(header, 'P2_X')
a(2) = eit_fxpar(header, 'P1_Y')
a(3) = eit_fxpar(header, 'P2_Y')

return, a
end