The ``units`` element information item
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``units`` element information items (referred to in this specification as
``units`` elements) are element information items in the CellML namespace
with local name equal to ``units``, and with either a ``model`` element or
a ``component`` element as their parent.

Specific information items
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1. Every ``units`` element MUST contain a ``name`` attribute in the empty
   namespace. The value of the ``name`` attribute MUST be a valid CellML
   identifier.

   a. Where the parent of the ``units`` element is a ``model`` element,
      the value of the ``name`` attribute MUST NOT be identical to the ``name``
      attribute of any other ``units`` element child of that ``model`` element, or
      of any ``import units`` element in the CellML infoset.

   b. Where the parent of the ``units`` element is a ``component`` element, the
      value of the ``name`` attribute MUST NOT be identical to the ``name``
      attribute of any other ``units`` element child of that ``component`` element.

   c. In any case, the value of the ``name`` attribute MUST NOT be equal to a
      cell in the name column of the :ref:`built-in-units` table.

2. A ``units`` element MAY contain a ``base_units`` attribute in the empty
   namespace. If present, the value of the ``base_units`` attribute MUST be
   equal to either ``yes`` or ``no``. If the attribute is present and equal to
   ``yes``, then the following paragraph does not apply.

3. A ``units`` element MAY contain one or more ``unit`` element children.