Factor      = production_name | token [ "…" token ] | Group | Option | Repetition .
Group       = "(" Expression ")" .
Option      = "[" Expression "]" .
Repetition  = "{" Expression "}" .
</pre>

<p>
Productions are expressions constructed from terms and the following
operators, in increasing precedence:
</p>
<pre class="grammar">
|   alternation
()  grouping
[]  option (0 or 1 times)
{}  repetition (0 to n times)
</pre>

	openDefers []*openDeferInfo
	// For open-coded defers, this is the beginning and end blocks of the last
	// defer exit code that we have generated so far. We use these to share
	// code between exits if the shareDeferExits option (disabled by default)
	// is on.
	lastDeferExit       *ssa.Block // Entry block of last defer exit code we generated
	lastDeferFinalBlock *ssa.Block // Final block of last defer exit code we generated