Excerpt from Lewis Carroll's book:
"Let me see:four times five is twelve,and four times six is thirteen,and four times seven is-oh dear! I shall never get to twenty at that rate!"
What mathematical machinery might the writer have had in mind when writing down the sentences above? Could one reengineer Carroll's thoughts behind? Is there any explanatory hint/link/allusion in Carroll's work or somewhere else? I know that Carroll was a mathematician. So, there must be something out there...