component_ref element information items (referred to in this
specification as component_ref elements) are element information items
in the CellML namespace with local name equal to component_ref.
Every component_ref element MUST contain an attribute information
item in the empty namespace and with local name component. The value
of this attribute MUST be a valid CellML identifier, and MUST match
the name attribute on a component element or an importcomponent element in the CellML infoset.
Every component_ref element MAY in turn contain zero or more
component_ref element children.
In addition, component_ref elements which are children of group
elements MUST contain at least one component_ref element child.