IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .
</pre>

<p>
If the type is present, all constants take the type specified, and
the expressions must be <a href="#Assignability">assignable</a> to that type.
If the type is omitted, the constants take the
individual types of the corresponding expressions.
If the expression values are untyped <a href="#Constants">constants</a>,

	// callback(x) calls goCallback(x)
	callbackMutex.Lock()
	callbackToken++
	i := callbackToken
	callbackFuncs[i] = f
	callbackMutex.Unlock()

	// Pass the address of i because the C function was written to
	// take a pointer.  We could pass an int if we felt like
	// rewriting the C code.
	C.callback(unsafe.Pointer(&i))

	callbackMutex.Lock()
	delete(callbackFuncs, i)
	callbackMutex.Unlock()
}

//export goCallback