Add First Class Closure rules.

Author: Crell

spec.md

@@ -1047,6 +1047,13 @@ $foo->bar(
 );
 ```
 
+### 7.1 Function callable references
+
+A function may be referenced in a way that creates a closure out of it, by providing `...` in place of arguments.
+
+If so, the `...` MUST NOT include any whitespace before or after.  That is, the correct format is `foo(...)`.
+
+
 ## 8. Anonymous Classes
 
 Anonymous Classes MUST follow the same guidelines and principles as closures