Let $f : X \to [-\infty, \infty]$ be a D3180: Basic function such that
| (i) | $f^+$ is the D1289: Basic function positive part of $f$ |
| (ii) | $f^-$ is the D1290: Basic function negative part of $f$ |
Then
\begin{equation}
|f| = f^+ + f^-
\end{equation}