The Lambert W function has two real branches: the principal branch and the secondary real branch: the former is denoted by $W_0$ or $W$, the latter by $W_{-1}$. How do we name them ?
For example, we can write: "Consider the solution $y = exp(-2 W(-x/2))$ where $W$ is the Lambert W function". In this case, the principal branch is implicitly assumed, so the issue is silently discarded. What can we write for the secondary case ? "Consider the solution $y = exp(-2 W_{-1}(-x/2))$ where $W_{-1}$ is ..."