An '''abstract structure''' defines a set of components, the relationships
between those components, and a set of constraints that hold for the structure.


** Description **

Each '''abstract structure''' implies a set of operations that may be performed
on the structure.  The structure definition, together with that set of
operations, is called an [abstract data types%|%abstract data type].  Tcl is
composed of a layered set of abstract structures:

   '''[string]''':   The fundamental structure.

   '''[word]''':   Defined in terms of [string].

   '''[list]''':   Defined in terms of [word].

   '''[dict]''':   Defined in terms of [list], with an additional constraint, and additional operations.  The operations it implies is a superset of the operations that [list] implies.

Additionally, there are two programmatic abstract structures:

   '''[command]''':   Defined in terms of '''[word%|%words]''' and [dodekalogue%|%substitutions].

   '''[script]''':   Defined in terms of [command].