Editing Diagrams/Dev/Arrows (section)

=== DelayedLeaf ===
At the risk of stating the obvious,
<code>Diagrams</code> (i.e. <code>DualTree</code>s) are created from the bottom (leaves) up, but <code>DTree</code>s
and <code>RTrees</code> from the top (root) down. Suppose we are creating an arrow using
diagrams functions and combinators. When we are finished we have essentially added
a subtree to a diagram. That subtree cannot use any of the informtion about
transforms and styles above it in the tree (they have not yet been created) so
the arrow is unable to use its final size and location (i.e. accumulated
transform) in its subtree. Brent's elegant solution was twofold:
# To hold off on creating the arrow by creating a new type of leaf, the <code>DelayedLeaf</code> in addition to <code>Prim b v</code> that we already had. The <code>DelayedLeaf</code> contains a funtion <code>DownAnnots v -> QDiagram b v m</code>, in other words a set of instructions about how to create the arrow once its <code>DownAnnots</code> are known.
# A modification of the <code>arrow'</code> function so that it returns a <code>DelayedLeaf</code>. The function wrapped in the <code>DelayedLeaf</code>, <code>delayedArrow</code> takes as parameter the <code>DownAnnots</code> and hence the styles and transforms.
The upshot is that an arrow gets to know the accumulated <code>DownAnnots</code> above it
to use it's conversion to a diagram.

Since creation of a <code>DTree</code> is top down, by the time
we reach a <code>DelayedLeaf</code> in the `DualTree` we already have all of the accumlated
transforms and styles that will be applied to the arrow. So the <code>DelayedLeaf</code>
can be expanded by running the function <code>DownAnnots v -> QDiagram b v m</code> on
the accumulated <code>DownAnnots</code> and recursively calling <code>fromDTree</code> to insert the
arrow into the <code>DTree</code>. It is import ant to note, though at this point the arrow
is essentially finished, all transforms and style have been applied. By the way, I'm pretty sure that at this point we could have removed the <code>scaleInv</code> wrapper from the head.

(As a side benefit, we are able to use all of the
styles that are applied above the arrow in the tree to do things like change
the color of the entire arrow with functions like <code>lc red</code>.)

There were still some enhancements we wanted to make,
https://github.com/diagrams/diagrams-lib/issues/162, but for the most part all
was well in Arrow land until ...