reorganize module

_archive/rhai_engine/rhaibook/language/global.md

@@ -0,0 +1,42 @@
+Automatic Global Module
+=======================
+
+{{#include ../links.md}}
+
+
+When a [constant] is declared at global scope, it is added to a special [module] called `global`.
+
+[Functions] can access those [constants] via the special `global` [module].
+
+Naturally, the automatic `global` [module] is not available under [`no_function`] nor [`no_module`].
+
+```rust
+const CONSTANT = 42;        // this constant is automatically added to 'global'
+
+{
+    const INNER = 0;        // this constant is not at global level
+}                           // <- it goes away here
+
+fn foo(x) {
+    x *= global::CONSTANT;  // ok! 'CONSTANT' exists in 'global'
+
+    x * global::INNER       // <- error: constant 'INNER' not found in 'global'
+}
+```
+
+
+Override `global`
+-----------------
+
+It is possible to _override_ the automatic global [module] by [importing][`import`] another [module]
+under the name `global`.
+
+```rust
+import "foo" as global;     // import a module as 'global'
+
+const CONSTANT = 42;        // this constant is NOT added to 'global'
+
+fn foo(x) {
+    global::CONSTANT        // <- error: constant 'CONSTANT' not found in 'global'
+}
+```