reserve x for set,
  p,q,r,s,t,u for ExtReal,
  g for Real,
  a for Element of ExtREAL;

theorem Th25:
  ].p,q.[ c= [.p,q.]
proof
A1: ].p,q.[ c= [.p,q.[ by Th22;
  [.p,q.[ c= [.p,q.] by Th24;
  hence thesis by A1;
end;
