The Ash-Choir of Mount Keldrith

Mount Keldrith sleeps only while it is sung to. Each night the monks of the Ash-Choir take turns chanting on the crater's rim. The volcano stirs during any moment when no monk is chanting. The Choir's abbess offers a bounty to whoever can name every silent span before the eruption-wardens post their watch.

Write silentSpans(chants, night).

• chants is a list of pairs [start, end] of integers. Each pair is one monk's chant and covers the half-open span from start up to but not including end. Every pair satisfies 0 <= start < end <= night. The list may be empty and is in no particular order. Chants may overlap, nest inside one another, repeat exactly, or touch end-to-start.
• night is a positive integer. The night is the half-open span from 0 up to night.

Return the silent spans as a list of pairs [start, end]. A silent span is a maximal half-open span within [0, night) that no chant covers.

• The list is sorted by start, ascending.
• Every span has start < end.
• No two returned spans overlap or touch.

If every moment of the night is covered, return an empty list.