SemigroupLaw

A semigroup in type F must satisfy two laws:

• '''closure''': ∀ a, b in F, append(a, b) is also in F. This is enforced by the type system.
• '''associativity''': ∀ a, b, c in F, the equation append(append(a, b), c) = append(a, append(b , c)) holds.