FunctionName = identifier .
FunctionBody = Block .
</pre>

<p>
If the function's <a href="#Function_types">signature</a> declares
result parameters, the function body's statement list must end in
a <a href="#Terminating_statements">terminating statement</a>.
</p>

<pre>
func IndexRune(s string, r rune) int {
	for i, c := range s {
		if c == r {
			return i
		}
	}
	// invalid: missing return statement