reorganize module

_archive/rhai_engine/rhaibook/ref/indexing.md

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+Indexing
+========
+
+```admonish tip.side "Tip: Non-integer index"
+
+Some data types take an index that is not an integer.
+For example, [object map](object-maps.md) indices are [strings](strings-chars.md).
+```
+
+Some data types, such as [arrays](arrays.md), can be _indexed_ via a Rust-like syntax:
+
+> _object_ `[` _index_ `]`
+>
+> _object_ `[` _index_ `]` `=` _value_ `;`
+
+Usually, a runtime error is raised if the index value is out of bounds or does not exist for the
+object's data type.
+
+
+Elvis Notation
+--------------
+
+The [_Elvis notation_](https://en.wikipedia.org/wiki/Elvis_operator) is similar except that it
+returns `()` if the object itself is `()`.
+
+> `// returns () if object is ()`  
+> _object_ `?[` _index_ `]`
+>
+> `// no action if object is ()`  
+> _object_ `?[` _index_ `]` `=` _value_ `;`