text: addressing the issue of the optical sum rule in the c−axis7 
  and in-plane conductivities 8–16 in overdoped, optimally 
  doped, and underdoped cuprates. The experimental re- 
  sults demonstrated, above all, outstanding achievements 
  of experimental abilities as these groups managed to de- 
  tect the value of the optical integral with the accuracy 
  of a fraction of a percent. The analysis of the change 
  of the optical integral between normal and SCS is even 
  more complex because one has to (i) extend NS data to 
  T < Tc and (ii) measure superﬂuid density with the same 
  accuracy as the optical integral itself. 
  
  The analysis of the optical integral showed that in over- 
  doped cuprates it deﬁnitely decreases below Tc, in con- 
  sistency with the expectations at weak coupling11. For 
  underdoped cuprates, all experimental groups agree that 
  a relative change of the optical integral below Tc gets 
  much smaller. There is no agreement yet about the sign 
  of the change of the optical integral : Molegraaf et al.8 
  and Santander-Syro et al.9 argued that the optical inte- 
  gral increases below Tc, while Boris et al.10 argued that 
  it decreases. 