Fusion Sequences are organized into a hierarchy of concepts.
Fusion's sequence traversal related concepts parallel Fusion's Iterator Concepts. Forward Sequence is the most basic concept. Bidirectional Sequence is a refinement of Forward Sequence. Random Access Sequence is a refinement of Bidirectional Sequence. These concepts pertain to sequence traversal.
The Associative Sequence concept is orthogonal to traversal. An Associative Sequence allows efficient retrieval of elements based on keys.