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Region-gain (unlike other automation) is specific to the region and independent of the source. Region::start() offset does not apply. When region-start is trimmed the region's envelope is modified (not just offset). The event-list is truncated. Any audio-region envelope does (and must) have a point exactly at the start and end of the region. truncate_start() can thus calculate the earliest position of valid events with the new length relative to the last event. The mathematical operator for that is subtraction, not distance.
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