theorem Th20:
  x in A^Fob iff for W st W in U_FMT x holds W meets A
proof
  thus x in A^Fob implies for W st W in U_FMT x holds W meets A
  proof
    assume x in A^Fob;
    then ex y st y=x & for W st W in U_FMT y holds W meets A;
    hence thesis;
  end;
  assume for W st W in U_FMT x holds W meets A;
  hence thesis;
end;
