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unit-element-item.rst

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The unit element information item

unit element information items (referred to in this specification as unit elements) are element information items in the CellML namespace with local name equal to unit, and with a units element as their parent.

Specific information items

  1. Every unit element MUST contain a units attribute information item in the empty namespace. The value of the units attribute MUST be a valid units reference, as defined in the :ref:`units-reference` section.
    1. The units element inclusion digraph SHALL be a conceptual digraph defined for the purpose of the constraint in the next paragraph, and SHALL contain one node for every units element in the CellML model. The units element inclusion digraph SHALL contain an arc from units element A to units element B if and only if units element A contains a unit element with units attribute value that is a units reference to units element B.
    2. The value of the units attribute MUST NOT be such that the units element inclusion digraph contains one or more cycles.
  2. A unit element MAY contain any of the following attribute information items in the empty namespace:
    1. The prefix attribute. If present, the value of the attribute MUST meet the constraints specified in the :ref:`interpretation-of-units` section.
    2. The offset attribute. If present, the value of the attribute MUST be a real number string. If the attribute is present and has value other than a real number string representing 0, then this unit element MUST NOT have any sibling unit elements, and the value of the exponent attribute, if present, MUST be a real number string representing 1.
    3. The multiplier attribute. If present, the value of the attribute MUST be a real number string.
    4. The exponent attribute. If present, the value of the attribute MUST be a real number string.