A Gaussian integer is by definition a complex number $\alpha=x+iy$, where $i=\sqrt{-1}$ and $x,y\in{\mathbb Z}$, and the set of Gaussian integers is denoted ${\mathbb Z}[i]$.

We call the four Gaussian integers $1, -1, i, -i$ Gaussian units.

The four numbers $\alpha, -\alpha, i\alpha, -i\alpha$ are called the associates of $\alpha$, (not to be confused with the word ``associate'' used for the inverse in $({\mathbb Z}/p{\mathbb Z})^\ast$, $p$ prime).

We say that $\pi$ is prime in ${\mathbb Z}[i]$ if and only if, for every factorization $\pi=\alpha\beta$, $\alpha$, $\beta$ $\in {\mathbb Z}[i]$, we must have $\alpha$, $\beta$ either associates of $\pi$ or Gaussian units.

Which of the following is **always true**?