Often, exceptions and deviations may help to ‘prove’ (in the correct sense of ‘test’) a rule, in that the degree
of surprise or outrage provoked by the deviation provides an indication of its importance, and the ‘normality’ of
the behaviour it prescribes.
(Watching the English; Kate Fox)
Should I read it as "...provides an indication of its importance, and (of) the 'normality' of the behaviour it prescribes."?