importunits element information items (referred to in this
specification as importunits elements) are element information items in
the CellML namespace with local name equal to units, which appear as
children of import elements.
Every importunits element MUST contain a name attribute in the
empty namespace. The value of the name attribute MUST be a valid CellML
identifier. The value of the name attribute MUST NOT be identical to
the name attribute of any other units element or importunits
element in the CellML infoset.
Every importunits element MUST contain a units_ref attribute in
the empty namespace. The value of the units_ref attribute MUST be a
valid CellML identifier. The value of the units_ref attribute MUST
match the value of the name attribute on a units element or
importunits element in the imported CellML infoset. The value of the
units_ref attribute MUST NOT match the value of the units_ref
attribute on any sibling importunits element.