Add support for MID function:

https://support.office.com/en-us/article/MID-MIDB-functions-d5f9e25c-d7d6-472e-b568-4ecb12433028

MID returns a specific number of characters from a text string, starting at the position you specify, based on the number of characters you specify.

MID(text, start_num, num_chars)
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