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.
The Unbounded Sequence concept is also orthogonal to traversal and associativity. A Unbounded Sequence allows out-of-bounds access.