A history entry is "additive" if it adds any new content to the input. An entry is "subtractive" if it removes any part of the input. An entry that replaces content is simultaneously additive and subtractive.
setPreviousEntry, canMergeWith
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A history entry is "additive" if it adds any new content to the input. An entry is "subtractive" if it removes any part of the input. An entry that replaces content is simultaneously additive and subtractive.