Fill in the blanks with the appropriate word, phrase, or symbol(s) from the following list.
$$\begin{array}{lllll}\text { inverse } & \text { domain } & \text { horizontal } & x & f(x) \\\text { vertical } & \text { range } & \text { one-to-one } & \text { composition } & y\end{array}.$$
When one variable is a function of another variable which in turn is a function of a third variable, we describe the relationship among such functions as a of ______ functions.