 We write \(f(\vec{n})\darrow\) to express that \(f(\vec{x})\) is defined at \(\vec{n}\) and additionally \(f(\vec{n})\darrow = m\) if \(f(\vec{n})\) is defined at \(\vec{n}\) and equal to \(m\). Otherwise we write \(f(\vec{n})\uarrow\) to express that \(f(\vec{x})\) is undefined at \(\vec{n}.\)