This optimizes the process of searching for a replacement edge to refrain from pushing forest edges down when a tree does not have any same-level non-forest edges. If a tree doesn't have any such edges, then there definitely isn't a replacement edge at that level, so we can avoid doing extra work.
Interestingly, with this optimization, ConnGraph reduces to a single Euler tour forest if the graph is a forest, with addEdge and removeEdge taking O(log N) (non-amortized) time with high probability.
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