theorem Th21:
  p is not zero implies Dir(p) is Element of ProjectivePoints(V)
proof
  assume p is not zero;
  then p in NonZero V by STRUCT_0:1;
  then Dir(p) in Class Proportionality_as_EqRel_of V by EQREL_1:def 3;
  hence thesis by Def5;
end;
