reserve Y for non empty set,
  a,b,c,d for Function of Y,BOOLEAN;
reserve Y for non empty set,
  a,b,c for Function of Y,BOOLEAN;

theorem Th9:
  a 'nand' (b '&' c) = 'not' (a '&' b '&' c)
proof
  a 'nand' (b '&' c) = 'not' (a '&' (b '&' c)) by th1;
  hence thesis by BVFUNC_1:4;
end;
