theorem for I1 being Element of U-InterpretersOf S1,
I2 being Element of U-InterpretersOf S2 st S2 is S1-extending &
X c= AllFormulasOf S1 & I1|(OwnSymbolsOf S1)=I2|(OwnSymbolsOf S1)
holds (X is I1-satisfied iff X is I2-satisfied) by Lm49;
