theorem Th1:
  f in F iff -f in --F
proof
  f in ExtREAL by XXREAL_0:def 1;
  hence f in F implies -f in --F;
  assume -f in --F;
  then
A1: ex w st -w = -f & w in F;
  - -f = f;
  hence thesis by A1;
end;
