Chain

WARNING - this is potentially an expensive operation

.asMatchable (not a runtime operation -- purely a compile-time cast satisfying Scala 3's Matchable safety check, since equals must take Any, which isn't itself Matchable) plus @unchecked on the pattern (the type test can only check erasure-level Chain[_, _] at runtime, not that other's own CellT/CoefficientT genuinely match this's -- accepted here exactly as it always has been: other's type parameters are assumed to line up with this's so other.collapseAll() can reuse this's own Ordering/Field givens, the same assumption every generic-class equals in this style makes).

Raw, uncollapsed entries: deferred arithmetic means a term like (a + a) can appear as two separate (a, coeff) pairs rather than one. Callers that need one coefficient per cell must collapse themselves (e.g. groupMapReduce).

Returns a string representation of the object.

The default representation is platform dependent.