Relativization with respect to formulas and its use in proofs of independence
Compositio Mathematica, Tome 20 (1968), pp. 88-106. 
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     author = {Henkin, Leon},
     title = {Relativization with respect to formulas and its use in proofs of independence},
     journal = {Compositio Mathematica},
     pages = {88--106},
     publisher = {Wolters-Noordhoff Publishing},
     volume = {20},
     year = {1968},
     mrnumber = {234812},
     zbl = {0155.02301},
     language = {en},
     url = {http://archive.numdam.org/item/CM_1968__20__88_0/}
}
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Henkin, Leon. Relativization with respect to formulas and its use in proofs of independence. Compositio Mathematica, Tome 20 (1968), pp. 88-106. http://archive.numdam.org/item/CM_1968__20__88_0/

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