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.

The outer loop is A=1 TO 3 for the three sections, the inner loop is B=
1 TO 4 for each line within each section. The tree is displayed 
symetrically around text column 20, so on each loop 20-A*B spaces are 
displayed to pad A*B stars to column 20, then another A*B-1 stars for 
the other side of the tree. This is optimised to 2*A*B-1 stars.

Finally, the two lines of the trunk are just displayed manually.
