theorem Th61:
  a '&' (b 'nor' c) = a '&' 'not' b '&' 'not' c
proof
  thus a '&' (b 'nor' c) =a '&' 'not' (b 'or' c) by Th2
    .=a '&' ('not' b '&' 'not' c) by BVFUNC_1:13
    .=a '&' 'not' b '&' 'not' c by BVFUNC_1:4;
end;
