reserve x for set;
reserve a,b,c,d for ExtReal;

theorem Th10:
  a in REAL & b >= a implies b in REAL or b = +infty
proof
  assume that
A1: a in REAL and
A2: b >= a;
  assume
A3: not b in REAL;
  b = -infty implies a = -infty by A2,Lm8;
  hence thesis by A1,A3,Lm10;
end;
