'The predicate is included in the subject of a true proposition'. Leibniz says this (in a letter to Arnauld) but of course his use of Latin signals that he is quoting an orthodox principle of the schools.
And the principle has nothing to do with the analytic/synthetic distinction. It has to do with the true/false distinction. If Socrates is the subject, and the subject is bald, and if the proposition states that the predicate 'is bald' is included in the subject (Socrates) then the proposition is true. Otherwise it is false.
What, then, is the analytic/synthetic distinction?