theorem Th44:
  p <= r & s < q implies ].r,s.] c= [.p,q.[
proof
A1: ].r,s.] c= [.r,s.] by Th23;
  assume that
A2: p <= r and
A3: s < q;
  [.r,s.] c= [.p,q.[ by A2,A3,Th43;
  hence thesis by A1;
end;
