luaL_wherevoid luaL_where (lua_State *L, int lvl);
Pushes onto the stack a string identifying the current position
of the control at level lvl in the call stack.
Typically this string has the format <chunkname>:<currentline>:.
Level 0 is the running function,
level 1 is the function that called the running function,
etc.
This function is used to build a prefix for error messages.