“If the length of the one is L the length of the other is L”. This is a form of various propositions but not itself a proposition But we could say: “If instead of ‘length’ we put ‘L’ || Let us put ‘L’ instead of ‘length’: then they have the same L.
     But it makes sense to say: “Let us consider propositions of the form “if A has the length L so has B”.”

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