Example 2

Tag 01MW, permanent. Cite it as [Bourbaki, Tag 01MW].

Topologie algébrique, chapter I, § 2. Applications étales

Let U be an open set of $\mathbf{C}$ . In order that a holomorphic function $f: U\rightarrow \mathbf{C}$ be an etale mapping, it is necessary and sufficient that its derivative nowhere vanish.

Bourbaki sets the proof directly after the statement, and the corpus does not separate them, so what is above is the statement and its proof together.

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