Each type info class representing a physical VHDL type is derived from
physical_info_base. A physical value is represented by the
number of basic units it consists of. This number is stored as a 64
bit integer. All units are represented by a ``unit index'' value where
0 is associated with the basic unit of the physical type. The next
unit defined by the physical type declaration is assigned 1, etc. An
array stores the name of the different units. Another array stores the
number of basic units each unit consists of. The arrays are indexed by
the ``unit index'' value.