Added 01substr
Added 02palyndrome.
Merge branch 'master' into exercise-3
Fix 02palyndrome. Add 03count-palyndromes.
Wording.
Add difference.
Add 05composition.
Add 06derive.
Improve the test infrastructure.
Add 07repeat.
Added sum and accumulate.
Added 10sqrt.scm
Merge branch 'exercise-3'
Remove some of the problems, intended for exercise 4.
Add a README.