reserve x,y,z,r,s for ExtReal;
reserve A,B for ext-real-membered set;

theorem Th34:
  x < y implies [.x,y.[ is left_end
proof
  assume
A1: x < y;
  then x in [.x,y.[ by XXREAL_1:3;
  hence inf [.x,y.[ in [.x,y.[ by A1,Th26;
end;
