theorem Th62:
  p [= q implies (r in [#p,q#] iff p [= r & r [= q)
proof
  assume p [= q;
  then [#p,q#] = {s where s is Element of L: p [= s & s [= q} by Def12;
  then r in [#p,q#] iff ex s being Element of L st r = s & p [= s & s [= q;
  hence thesis;
end;
