theorem Th13:
  not c1,c2,c5 are_collinear &
  c1,c5,c6 are_collinear & c1,c5,c7 are_collinear
  implies c5,c6,c7 are_collinear
  proof
    assume that
A1: not c1,c2,c5 are_collinear and
A2: c1,c5,c6 are_collinear and
A3: c1,c5,c7 are_collinear and
A4: not c5,c6,c7 are_collinear;
    now
      thus for v100,v2 being Element of PCPP holds
        v2,v100,v100 are_collinear by COLLSP:2;
      not c5=c1 or c1,c2,c5 are_collinear by COLLSP:2;
      hence
      for v0 being Element of PCPP holds
      not c1,c5,v0 are_collinear or v0,c6,c7 are_collinear
        by COLLSP:3,A1,A2,A3;
    end;
    hence contradiction by A4;
  end;
