Function r1c1_to_a1 

pub fn r1c1_to_a1(s: &str, at: CellRef) -> Result<Option<String>, CellError>

Converts an R1C1 reference text to A1 (for INDIRECT(…, FALSE)). Ok(None) means the text is not R1C1; Err means it is R1C1 but points past the sheet.