theorem
  p '&' q |-| q '&' p
proof
A1: {q,p} |-| {q '&' p} by Th31;
  {p '&' q} |-| {q,p} by Th31;
  then {p '&' q} |-| {q '&' p} by A1;
  hence thesis by Th29;
end;
