Editing Diagrams/Dev/Arrows (section)

=== ScaleInv ===
The original semantics we choose for arrows was for only the shaft length to
change under scaling transformations. This was done not only because we
wanted the head size (from here on what ever I say about head applies to
tail as well) to remain constant, but becuase non-uniorm scalings would cause
the head to point in the wrong direction. See http://projects.haskell.org/diagrams/doc/manual.html#scale-invariance. It
turned out that our solution (wrapping the head in the `ScaleInv` wrapper) was
not enough. Once an arrow was stroked, it would not behave correctly under scales.
We solved the problems of keeping the head size constant and making the arrow
point in the correct direction, but now a scaling could cause the head and shaft
to separate. As explained in https://github.com/diagrams/diagrams-lib/issues/112.
Hence the birth of delayed subtrees.