theorem Th37:
  p <= r & s <= q implies ].r,s.[ c= [.p,q.]
proof
A1: ].r,s.[ c= [.r,s.] by Th25;
  assume that
A2: p <= r and
A3: s <= q;
  [.r,s.] c= [.p,q.] by A2,A3,Th34;
  hence thesis by A1;
end;
