Avoid usedElements around code that modifys that value (#3488)
Avoid usedElements around code that modifys that value (#3488)

Followup to #3486, I wonder if it isn't a little more clear this way,
which avoids the confusion of usedElements being changed
while we are using it.

In general I think it's best to only use usedElements in the most
internal methods, and to call size() otherwise.
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