How it works
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If you look at the Christmas tree split in half vertically you can see 
that the length of each horizontal line is: 1,2,3,4, 2,4,6,8, 3,6,9,12. 
This is a simple multiplication sequence, one two is two, two twos are 
four, three twos are six, etc. There are three sections and four 
horizontals in each section, so a simple 3 times 4 FOR/NEXT loop 
generates the width of each line.

We create these two FOR/NEXT loops with R4 as the outer loop counting
sections and R3 as the inner loop counting branches.

At the beginning of the section loop we increment the section counter
and use that as the value to add to the number of leaves on each branch.
So, in the first section we add 1 on each branch giving 1,2,3,4; on the
second section we add 2 giving 2,4,6,8; etc.

We then pad to 20 minus the branch size to align the centre of the branch
to text column 20, then output 2*N-1 stars for the leaves to output both
sides of the branch. We can easily calculate 2*N-1 by shifting left (ASL)
and decrementing (DEC).

We count to do three sections. After this we do the trunk. This is a simple loop counting two slices, outputting 18 spaces for padding and
the three stars for the trunk.

The final bit of code is a subroutine to optimise the multiple character
output.

The whole code starts with a Unix loader header to instruct the loader
how to load the code, and a subroutine to output a character to the
Unix STDOUT output stream.
