theorem Th3:
  not o on A & not o on B & y on B implies ex x st x on A & IncProj
  (A,o,B).x = y
proof
  consider X such that
A1: o on X & y on X by INCPROJ:def 5;
  consider x such that
A2: x on X and
A3: x on A by INCPROJ:def 9;
  assume ( not o on A)& not o on B & y on B;
  then IncProj(A,o,B).x = y by A1,A2,A3,PROJRED1:def 1;
  hence thesis by A3;
end;
