theorem
  ( p 'or' q ) <=> ( q 'or' p ) in TAUT(A)
proof
  set P = p 'or' q;
  set Q = q 'or' p;
  P => Q in TAUT(A) & Q => P in TAUT(A) by Th8;
  then ( P => Q ) '&' ( Q => P ) in TAUT(A) by Lm4;
  hence thesis by QC_LANG2:def 4;
end;
