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

theorem Th13:
  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,Lm9;
  hence thesis by A1,A3,Lm10;
end;
