I wrote:
Generally the tangent of the sum of several angles is given by [insert mathematical expression here]. That is proved by an easy induction on n once one has the case of two angles.
Some sort of copyeditor changed it to this:
Generally the tangent of the sum of several angles is given by [insert mathematical expression here]. It is proved by an easy induction on n once one has the case of two angles.
"That" was changed to "it." My efforts to correct a gross error by the copyeditor (or whatever they were) elsewhere in the paper distracted me from the issue of the merits or demerits of that particular change, and the paper was published with "it" instead of "that."
Hardly a disaster, but I am not as comfortable with "it" as with "that."
Am I right or am I being too nitpicking?