Existing Wording: The code system hierarchy is a single tree, where concepts have only one parents.

Comment:

The very next sentence is: Some code system concepts have more than one parent, so the parent property may repeat. Subsumes extension must be used.

This is a confusing message - if a hierarchy is defined for a code system, it can only have one - if a code system supports more than one hierarchy 'type', leave the value null?

This should be clarified

Summary:

Clarify support for multiple parents