theorem Th29:
  for r being Real st PM is triangle holds Ball(x,r) in
  Family_open_set(PM)
proof
  let r be Real;
  assume PM is triangle;
  then
  for y st y in Ball(x,r) holds ex p st p>0 & Ball(y,p) c= Ball(x,r) by Th27;
  hence thesis by Def4;
end;
