Sequence Diagram Imports do not correctly position imported content

[markallanson]

Imports within sequence diagrams appear to hoist the imported content to the top of the sequence diagram.

Using the following example files:

a.d2

a -> b
...@b

b.d2

b -> c

I would expect the generated diagram to be equivalent to:

a -> b
b -> c

but instead it generates a diagram equivalent to:

b -> c
a -> b