theorem
  a 'or' (a 'nor' b) = a 'or' 'not' b
proof
  thus a 'or' (a 'nor' b) =(a 'or' 'not' a) '&' (a 'or' 'not' b) by Th62
    .=I_el(Y) '&' (a 'or' 'not' b) by BVFUNC_4:6
    .=a 'or' 'not' b by BVFUNC_1:6;
end;
