pro react_to_every_particle_change, old, new
@common_blocks.inc
if old eq new then return
if (new mod old) eq 0 then begin  ; some fewer points. we can fix that
   stride = fix(new/old)
   print, 'react:', stride, new, old
   for i=0,N_elements(*parti.names)-1 do $ 
      if parti.read[i] gt 0 then begin
      *parti.d[i] = (*parti.d[i])[0:*:stride]
   endif
   return
endif

for i=0,N_elements(*parti.names)-1 do begin
   parti.d[i] = ptr_new(0.)
   parti.read[i] = 0
endfor

return
end