AvalonEdit - Event when Caret enters ReplaceableTextElement?

Hello - I'm attempting to dynamically populate a CompletionWindow based on the current Caret position. I.e, if a user selects to insert a Snippet from a CompletionWindow, and that Snippet has one or more SnippetReplaceableTextElement, open a new CompletionWindow (with contextual items) when the Caret enters the SnippetReplaceableTextElement(s).

Is this possible? What approach might I take to accomplish my intended behavior?

Thanks.