map_variables element information items (referred to in this
specification as map_variables elements) are element information items
in the CellML namespace with local name equal to map_variables, and
which appear as a child of a connection element.
Each map_variables element MUST contain a variable_1 attribute in
the empty namespace. The value of the variable_1 attribute MUST be a
valid CellML identifier. The value of this attribute MUST be equal to the
name attribute on a variable element child of the component
element referenced by the component_1 attribute on the map_components
element which is a sibling of this element.
Each map_variables element MUST contain a variable_2 attribute in
the empty namespace. The value of the variable_2 attribute MUST be a
valid CellML identifier. The value of this attribute MUST be equal to the
name attribute on a variable element child of the component
element referenced by the component_2 attribute on the map_components
element which is a sibling of this element.