function indexAt(str) {
  let ret = 0;
  for(let i = 0; i !== str.length; ++i) ret = 26 * ret + str.charCodeAt(i) - 64;
  return ret - 1;
}
translate letter in A1-tag to number
@date 2019-10-10
@export
@param {string} str "AA" in A1-tag "AA1"
@returns {number}