Preferred arrow directions in models of composite objects

[mhlr]

Arrow in models of composite systems tend to have the oppsite directions in programming languages (both OO & Functional) and relational models. In programing landuages the parent knows about its children while in relational models the children tend to know about their parent.
There are several reasons for this

• relational models do not support multivalued attributes
• it makes it easy to grow a tree nondestructively in the relational model
• relational models have indexes andbult in operations like join that make it possible to find all the descendants of a node even when the arrows point upwards.

What is the preferred way in Catlab?

• parent has a collection of children
• each child has a parent