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 'nor' (a 'or' b) = 'not' (a 'or' b)
proof
  thus a 'nor' (a 'or' b) = 'not' (a 'or' a 'or' b) by Th34
    .= 'not' (a 'or' b);
end;
