pro get_hist_var,xx,count,back,expt,dist,dl,dh,flux,eflux,texpt $
,dlow=dlow,dhigh=dhigh,ctonth=ctonth,sel=sel
;-
; 
; written by wqd, March 8, 1996
;+
if n_params() eq 0 then begin
print,'CALLING SEQUENCE -- get_hist_var,xx,count,back,expt,dist,dl,dh,flux,eflux,texpt'
print,',dlow=dlow,dhigh=dhigh,ctonth=ctonth,sel=sel'
return
endif
if n_elements(ctonth) eq 0 then ctonth=3.
nsel=n_elements(sel)
if  nsel ne 0 then begin
        x=xx(sel)
        rcount=count(sel)
        rback=back(sel)
        rexpt=expt(sel)
endif else begin
        nsel=n_elements(xx)
        x=xx
        rcount=count
        rback=back
        rexpt=expt
endelse
flux=[-999]
eflux=[-999]
texpt=[-999]
dist=[-999]
if n_elements(dlow) eq 0 then dl=x(0) else dl=dlow
if n_elements(dhigh) eq 0 then dhigh=x(nsel-1) 

dh=[-999]
db=0. & cb=0 & bb=0. & eb=0.
for k=0L, nsel-1 do begin
        db=db+x(k)*rcount(k)
        cb=cb+rcount(k)
        bb=bb+rback(k)
        eb=eb+rexpt(k)
;       if cb ge ctonth or k eq (nsel-1) then begin 
        if imdiv(cb-bb,sqrt((cb > bb) > 5.)) ge ctonth or k eq (nsel-1) then begin 
                if k ne (nsel-1) then div=(x(k)+x(k+1))*0.5 else div=dhigh
                dh=[dh,div]
                dl=[dl,div]
                dist=[dist,db/cb]
                flux=[flux,(cb-bb)]
                eflux=[eflux,sqrt(cb > bb)]
                texpt=[texpt,eb]
                db=0 & cb = 0 & bb=0 & eb=0
        endif
;if k gt 60 then stop
endfor
dl=dl(0:n_elements(dist)-2)
dh=dh(1:*)
dist=dist(1:*)
flux=flux(1:*)
eflux=eflux(1:*)
texpt=texpt(1:*)
end