Example

Tag 03RY, permanent. Cite it as [Bourbaki, Tag 03RY].

Theory of Sets, chapter I, § 1. Terms and relations

The symbol $\tau_x(\in xy)$ represents the assembly

\tau\in\square y.

Let $A$ and $B$ be assemblies and let $x$ be a letter. The assembly obtained by replacing $x$ , wherever it occurs in $A$ , by the assembly $B$ is denoted by $(B\mathbin{|}x)\,A$ (read : $B$ replaces $x$ in $A$ ). If $x$ does not appear in $A$ , then $(B\mathbin{|}x)\,A$ is identical with $A$ ; in particular,

(B\mathbin{|}x)\,\tau_x(A)

is identical with $\tau_x(A)$ .

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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