Indeterminate form (spec §6.2): mathematically undefined forms (0/0 etc.) that exist only in the symbolic layer;
they can take part in later simplification; when collapse to the numeric layer fails they become Undefined.
Value type (spec §5): covers the value forms of each layer of the three-world architecture —
the symbolic layer (Expr/Symbol/Indeterminate), the numeric layer (Number), and the host layer
(Bool/String/Error, etc.). Array is a variable-length heterogeneous sequence (v2.1, spec §11.3);
Dict/Set are variable host collections keyed/elemented by immutable hashable ValueKeys (spec §4.6/§11.6).
Result/Error carry a structured Error as a message string (the structured enum from spec §16.1 is deferred to a later release).
Hashable key for Dict/Set (spec §11.6): a value-semantic, immutable subset of Value —
numbers (canonicalized), strings, chars, bools, and symbol/expr handles.