Why not possible to do a tensor sum of a concern with another concern?

[SergeStinckwich]

This is possible to do a tensor sum (integrate:) a model with a concern:

model := KEModel new.
concernA := KEModelPart new.
concernB := KEModelPart new.
model integrate: concernA.
model integrate: concernB.

but not the reverse:

concernA integrate: concernB.

because integrate: is not defined on KEModelPart