assistant
[EXECUTING SPECIFIC TASK FROM NONTRIVIAL TASK-5-1]
Task: Implement span_merge.
Description: Given booking spans as a start and an end each, combine every span that overlaps another, or merely touches it end to start, into a single span. Return what is left ordered by start. Spans arrive in no particular order.
Function signature: def span_merge(spans: list) -> list
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