Hi Alan,

The "A" prefix is the temporal annotation.  The technical term is the 
SignSpelling Sequence, which is used primarily for sorting.  The "A" 
prefix is an ordered list of symbols.

You can drop the "A" prefix if you only care about visual construction.

If we consider a longer example, such as "world":

The "A" prefix always ends at an L, M, or R; as these are the markers 
for the start of a signbox in the left, middle, or right lanes 
respectively.  In this example, the "A" prefix contains 4 symbol keys. 
"A S18701 S1870a S2e734 S20500".

The rest of the example contains the spatial annotation: symbols with 

A few quick rules
The x-axis is negative on the left and positive on the right.
The y-axis is negative on the top and positive on the bottom.
The symbols are positioned by the top-left of the symbol glyph.
The coordinate (0,0) is the center of a sign.

The first part "M18x33" tells us that the signbox is in the middle lane 
with a max coordinate of (18,33).

The rest is simply symbols with coordinates:
S1870a n11x15 S18701 n18xn10 S20500 8xn4 S2e734 0xn32

The same string with explicit coordinates:
S1870a at (-11,15)
S18701 at (-18,-10)
S20500 at (8,-4)
S2e734 at (0,-32)

The complete "Hello world." example is online with the various notations: