Building Simple Parse Trees
The notation $S\to(L)|a$ is shorthand to $S\to(L)$ and $S\to a$, so you can rewrite the grammar in your equivalent form; I don't see how this makes things any different, since the two grammars are exactly the same.

The first step in writing a parse tree is coming up with a derivation. In the derivation below, I use boldface to indicate the non-terminal being expanded.
$$
\begin{align*}
\mathbf{S}&amp;\to(\mathbf{L}) \\ &amp;\to(\mathbf{L},S) \\ &amp;\to (\mathbf{S},S) \\ &amp;\to (a,\mathbf{S}) \\ &amp;\to (a,(\mathbf{L})) \\ &amp;\to (a,(\mathbf{L},S)) \\ &amp;\to (a,(\mathbf{S},S)) \\ &amp;\to (a,(a,\mathbf{S})) \\ &amp;\to (a,(a,a))
\end{align*}
$$
There are several different derivations which "feel" the same. For example, in this derivation I always expanded the left-most non-terminal, but you could just as well expand any other non-terminal. When you draw the derivation tree, all these derivations give rise to the same tree.

Here is the actual derivation tree, drawn by Rick Decker: