I am reading "Elements of Mathematics Theory of Sets" by Nicolas Bourbaki. (English translation.)
In the following remark, what is the symbol $Z$?
I cannot understand what the author wants to say at all.
Remark. When an abbreviating symbol $\Sigma$ is introduced, by means of a definition, to represent a certain assembly, the (usually tacit) convention is made of representing the assembly obtained by substituting an assembly $B$ for a letter $x$ in the original assembly, by the symbol obtained by the replacing the letter $x$ in $\Sigma$ by the assembly $B$ (or, more often, by an abbreviating symbol representing the assembly $B$).
For example, having defined what assembly is represented by the symbol $E\otimes F$, where $E$ and $F$ are letters - an assembly which, incidentally, contains other letters besides $E$ and $F$ - the symbol $Z\otimes F$ can be used without further explanation.
By the way, I recently bought several books by Nicolas Bourbaki. (English translation.)
Sadly, these books seem to be beyond my understanding.
They seem to have been a complete waste of money.
I wanted to read "Algebra" and "General Topology".
