theorem Th40:
  p < r & s <= q implies [.r,s.[ c= ].p,q.]
proof
A1: [.r,s.[ c= [.r,s.] by Th24;
  assume that
A2: p < r and
A3: s <= q;
  [.r,s.] c= ].p,q.] by A2,A3,Th39;
  hence thesis by A1;
end;
