Update replacement.tex

Added a footnote explaining the meaning of `term' in this context.

Author: timbutton

content/set-theory/ordinals/replacement.tex

@@ -38,7 +38,7 @@
 
 On first encounter, however, this is quite a tangled formula. The
 following quick consequence of Replacement probably gives a
-\emph{clearer} expression to the intuitive idea we are working with:
+\emph{clearer} expression to the intuitive idea we are working with:\footnote{A term is an expression which picks out exactly one object (on any completion of its free variables). For example, ``$\emptyset$'' is a term which picks out the empty set; ``$\{x\}$'' is a term which picks out $x$'s singleton (whatever $x$ might be); ``$x \cup y$'' is a term which picks out the union of $x$ and $y$ (whatever they might be).}
 
 \begin{cor}
 For any term $\tau(x)$, and any set $A$, this set exists:
@@ -69,4 +69,5 @@
 f(x)}$.} Replacement, by contrast, is a \emph{powerful} addition to
 our axioms, as we will see in \olref[sth][replacement][]{chap}.
 
-\end{document}
+
+\end{document}