ABC is basically trivial

To understand the ABC music  notation,  imagine  you're  a  beginning
music student who has pecked out a tune on an instrument and wants to
put it on paper.  The first thing you might think of is writing  down
the notes by their letter names:

   CCCDEEDEFG

This might be recognized by some people immediately.  To make it more
recognizable, your second guess might be to  indicate  how  long  the
notes  are.  How would you do this?  Well, one way might be to follow
each note by its length.  This tune is now:

   C2C2CDE2EDEFG4

Most people will recognize the tune already.  The next thing we might
do to make it more readable is to break it up with spaces to show the
rhythm better.  Also, since the keyboard has a character  that  looks
like a bar line, why not use it?  Now the tune is:

   | C2C2 CDE2 | EDEF G4 |

You now know maybe a quarter of what you need to  to  read  and write
tunes  in  ABC.  There's a lot more, such as how to indicate octaves.
CDEFGAB are the lower octave and cdefgab are the upper octave on  the
staff, and there's notation for other octaves. There are header lines
where you can put information like title, composer, meter and key.

I have some more documents online:

http://trillian.mit.edu/~jc/music/abc/doc/ABCtrivial.txt  (this file)
http://trillian.mit.edu/~jc/music/abc/doc/ABCintro.txt   (text intro)
http://trillian.mit.edu/~jc/music/abc/doc/ABCprimer.html  (web intro)
http://trillian.mit.edu/~jc/music/abc/doc/ABCtutorial.html (tutorial)