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

theorem Th11:
  not -infty in REAL
proof
A1: {0,REAL} in {{0,REAL},{0}} & REAL in {0,REAL} by TARSKI:def 2;
  assume -infty in REAL;
  hence contradiction by A1,XREGULAR:7;
end;
