let seq be ExtREAL_sequence; :: thesis: for a being R_eal st ( for n being Nat holds seq . n <= a ) holds
sup seq <= a
let a be R_eal; :: thesis: ( ( for n being Nat holds seq . n <= a ) implies sup seq <= a )
assume A1: for n being Nat holds seq . n <= a ; :: thesis: sup seq <= a

now

let x be ext-real number ; :: thesis: ( x in rng seq implies x <= a )
assume x in rng seq ; :: thesis: x <= a
then consider z being set such that
A2: z in dom seq and
A3: x = seq . z by FUNCT_1:def 5;
thus x <= a by A1, A2, A3; :: thesis: verum

end;

then a is UpperBound of rng seq by XXREAL_2:def 1;
hence sup seq <= a by XXREAL_2:def 3; :: thesis: verum