Add a directory for exercise 4.
Added 04difference.
Merge branch 'master' into exercise-4
A little bit of housekeeping.
More preparations.
Merge branch 'master' into exercise-4
Added a README.
Add three new problems.
Add 10prime.scm.
Add sin and cos.
Add 11reverse.scm.
Add V and C.
Reordering.
Minor: Newline at EOF.
Add 14sqrt.
Merge branch 'exercise-4'