He thought he saw a Garden-Door
That opened with a key:
He looked again, and found it was
A Double Rule of Three:
‘And all its mystery,’ he said,
‘Is clear as day to me!’
Lewis Carroll, Sylvie and Bruno and Sylvie and Bruno Concluded
After going in the wrong direction for a while, I understood that this is not about applying the Bellman’s Rule twice. I think that Lewis Carroll (like Charles T. Brooks) quite probably referred to cross multiplication.
Then again, I am not shure whether my direction was that wrong. Carroll could have used Double Rule of Three with a double meaning:
※ the extension to the rule of three for cross-multiplication and
※ the double application of the Bellman’s rule.
More on the Rule of Three: Alfred Crowquill (pen name of Alfred Henry Forrester), Comic Arithmetic, London 1843, p. 96
2022-05-31, updated: 2022-06-12 (<!MG007>MG007)