theorem
  for G being IncProjSp, a,b,c being POINT of G st c on a*b & a<>c holds
  b on a*c
proof
  let G be IncProjSp, a,b,c be POINT of G;
  assume that
A1: c on a*b and
A2: a<>c;
  now
    assume
A3: a<>b;
    then a on a*b by Th16;
    then a*b = a*c by A1,A2,Th16;
    hence thesis by A3,Th16;
  end;
  hence thesis by A2,Th16;
end;
