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
  a 'nand' (b 'eqv' c) = a 'imp' (b 'xor' c)
proof
  a 'nand' (b 'eqv' c) = 'not' (a '&' (b 'eqv' c)) by th1
    .= ('not' a) 'or' 'not' (b 'eqv' c) by BVFUNC_1:14
    .= ('not' a) 'or' 'not' 'not' (b 'xor' c) by Th12
    .= ('not' a) 'or' (b 'xor' c);
  hence thesis by BVFUNC_4:8;
end;
