The type BaseIndex also models indices into list boxes, entry
widgets etc., but for editors only the constructor
IndexPos is important:
The class HasIndex models which way of indexing a position inside a widget is valid. For text widgets, we have
so we can just specify the index as a position (i.e. a row and column number, starting from 0), but we can also specify the index by the constant EndOfText (only constructor of the data type EndOfText), or by its screen coordinates (Pixels), or by modifying one of the other indices:
Index modifiers (only for editors) are modelled by type Editor.html#Editor.IndexModifierIndexModifier, and specify things like `three words forward', or a `one line back'.