SUPPLEMENTAL INDENTUREDated as of August 9, 2017COMMONWEALTH EDISON COMPANYtoBNY MELLON TRUST COMPANY OF ILLINOISandD.G. DONOVANTrustees Under Mortgage Dated July 1, 1923,and CertainIndentures Supplemental TheretoProviding for Issuance ofFIRST...