\begin{definition}[Definition:Real Function/Two Variables/Substitution for y]
Let $S, T \subseteq \R$ be subsets of the set of real numbers $\R$.
Let $f: S \times T \to \R$ be a (real) function of two variables:
:$z = \map f {x, y}$
Then:
:$\map f {x, a}$
means the real function of $x$ obtained by replacing the independent variable $y$ with $a$.
In this context, $a$ can be:
:a real constant such that $a \in T$
:a real function $\map g x$ whose range is a subset of $T$.
\end{definition}