let x be ExtReal; :: thesis: inf {x} = x
A1: for z being LowerBound of {x} holds z <= x
proof
let z be LowerBound of {x}; :: thesis: z <= x
x in {x} by TARSKI:def 1;
hence z <= x by Def2; :: thesis: verum
end;
x is LowerBound of {x} by Th2;
hence inf {x} = x by A1, Def4; :: thesis: verum