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.
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 Units reference section.
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.
The value of the units attribute MUST NOT be such that the units
element inclusion digraph contains one or more cycles.
A unit element MAY contain any of the following attribute information
items in the empty namespace:
The prefix attribute. If present, the value of the attribute MUST
meet the constraints specified in the Interpretation of units section.
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.
The multiplier attribute. If present, the value of the attribute
MUST be a real number string.
The exponent attribute. If present, the value of the attribute
MUST be a real number string.