-/// The default functor implementation forwards to a getSparseSetKey() method
-/// on the object. It is intended for sparse sets holding ad-hoc structs.
+/// A simple type declaration, SparseSet<Type>, handles these cases:
+/// - unsigned key, identity index, identity value
+/// - unsigned key, identity index, fat value providing getSparseSetIndex()
+///
+/// The type declaration SparseSet<Type, UnaryFunction> handles:
+/// - unsigned key, remapped index, identity value (virtual registers)
+/// - pointer key, pointer-derived index, identity value (node+ID)
+/// - pointer key, pointer-derived index, fat value with getSparseSetIndex()
+///
+/// Only other, unexpected cases require specializing SparseSetValTraits.
+///
+/// For best results, ValueT should not require a destructor.