code: function cellref (ref) {
    if (R1C1.test(ref)) {
      return convertR1C1toA1(ref)
    }
  
    if (A1.test(ref)) {
      return convertA1toR1C1(ref)
    }
  
    throw new Error(`could not detect cell reference notation for ${ref}`)
  }
docs: Auto detect notation used and convert to the opposite notation
  
  @param   {String} ref
  @returns {String}
  @throws  {Error}