Hi:

We needed those methods for a college project and we implemented them 
with GSL, I know Euler and Improved Euler are merely didactic methods 
but maybe some students (like us) could avoid reinventing the wheel.

On the other hand, 4 step Adams-Bashford-Moulton method could be  usable 
in a real application. We used Runge-Kutta to obtain the remainig three 
initial steps.

I attached the 3 files just in case you want to include them in the GSL 
distribution

Cheers

Alfonso Acosta

PS: The three methods were tested without step control, so we are not 
sure wether the error we provide is correct or not.