theorem Th25:
  x is Element of the Lines of ProjHorizon(AS) implies [x,2] is
  LINE of IncProjSp_of(AS)
proof
  assume x is Element of the Lines of ProjHorizon(AS);
  then ex X st x=PDir(X) & X is being_plane by Th15;
  hence thesis by Th23;
end;
