rarely for me. I usually let the wind blow where it wills... however, if i have just played a hymn, then almost always it starts from there and wanders off.
That I know of, there is nothing approaching legislation about (or upon) this. (Don't anyone tell the people in Rome, though!) It seems to me that what one nearly always encounters, by way of convention and precedent, is an improvisation 'on' such and such; but, if one were to be so pretentious as to improvise 'upon' such and such, it had ought to be awfully good!
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