/// <summary> 
/// Transforms excel column string indices (e.g. A, B, Z, AA, CD) to index number (starting with A = 1).
/// </summary> 
/// <param name="columnAdress">param description here</param>
let colAdressToIndex (columnAdress : string) =
    let length = columnAdress.Length
    let mutable sum = 0u
    for i=0 to length-1 do
        let c = columnAdress.[length-1-i] |> System.Char.ToUpper
        let factor = 26. ** (float i) |> uint
        sum <- sum + ((uint c - 64u) * factor)
    sum