;**************************************************************************
function ERRDIV,X0,DX0,Y0,DY0,print=print,unc=unc
;
if n_params(0) lt 2 then begin
   print,' '
   print,' * ERRDIV -- return statistical error on X/Y'
   print,' *    calling sequence: UNC=ERRDIV(X,dX,Y,dY)'
   print,' *       X: numerator'
   print,' *      dX: uncertainty on numerator'
   print,' *       Y: denominator'
   print,' *      dY: uncertainty on denominator'
   print,' *  KEYWORDS:'
   print,' *     PRINT: set to print result to screen'
   print,' *       UNC: set for relkative uncertainty. E.g., UNC=0.1 for 10% error'
   print,' *'
   print,' *   call as UNC=ERRDIV(X,Y) to use sqrt(X),sqrt(Y) as errors'
   print,' '
   return,-999
   endif
;
x=x0 & dx=dx0
fx=float(x)
if n_elements(unc) eq 0 then unc=0
if n_elements(unc) gt 0 then unc=unc(0)
unc=abs(unc)
case 1 of
   unc gt 1.: begin
      if unc lt 100. then unc=unc/100. else unc=0   ;change percent to fraction 
      end
   else:
   endcase
case 1 of
   unc gt 0: begin
      fy=float(dx)
      dx=fx*unc
      dy=fy*unc
      end
   n_params(0) eq 2: begin
      fy=float(dx)
      dx=sqrt(fx)
      dy=sqrt(fy)
      end
   n_params(0) eq 3: begin
      y=y0
      fy=float(y)
      dy=sqrt(fy)
      end
   else: begin
      y=y0
      dy=dy0
      fy=float(y)
      endelse
   endcase
;
err=SQRT(DX*DX+(fX*DY/fY)*(fX*DY/fY))/fY
if keyword_set(print) then print,fx/fy,'+\-',err
return,err
end