theorem
  M,v |= p '&' q => q '&' p & M |= p '&' q => q '&' p
proof
  now
    let v;
A1: M,v |= p & M,v |= q implies M,v |= q '&' p by ZF_MODEL:15;
    M,v |= p '&' q implies M,v |= p & M,v |= q by ZF_MODEL:15;
    hence M,v |= p '&' q => q '&' p by A1,ZF_MODEL:18;
  end;
  hence thesis;
end;
