Question on DeltaR and vectors in rdf


your suggestions of 7/2/19 helped quite a bit wrt to rdf. thanks.
however i am still missing something in one of the examples you gave.
i am using root 6.18 now.

df = df.Filter("nMuon > 0 && nElectron > 0", "Select only valid events")\
       .Define("idx", "Combinations(Muon_phi, Electron_phi)")\
       .Define("dR", "DeltaR(Take(Muon_phi, idx[0]), Take(Electron_phi, idx[1]),\
                                          Take(Muon_eta,idx[0])"), Take(Electron_eta,idx[1]))
       .Define("minIdx", "ArgMin(dR)")\
       .Define("dRMin", "dR[minIdx]")

so lets say i have 2 muons and 2 electrons, and i am trying to find the smallest dR between the

a.  i am not sure what the idx should be with the Takes in defining DeltaR.  i want to loop
over all the possible combinations and i dont think the way i defined dR does that.  
or am i wrong?