Details in comments and documentation

lstate.h

@@ -334,6 +334,12 @@ struct lua_State {
 
 /*
 ** Union of all collectable objects (only for conversions)
+** ISO C99, 6.5.2.3 p.5:
+** "if a union contains several structures that share a common initial
+** sequence [...], and if the union object currently contains one
+** of these structures, it is permitted to inspect the common initial
+** part of any of them anywhere that a declaration of the complete type
+** of the union is visible."
 */
 union GCUnion {
   GCObject gc;  /* common header */
@@ -347,6 +353,11 @@ union GCUnion {
 };
 
 
+/*
+** ISO C99, 6.7.2.1 p.14:
+** "A pointer to a union object, suitably converted, points to each of
+** its members [...], and vice versa."
+*/
 #define cast_u(o)	cast(union GCUnion *, (o))
 
 /* macros to convert a GCObject into a specific value */