isPrefix([ ], L) = true
Example: isPrefix([ ], [2,4,6]) = true.
isPrefix(A, [ ]) = false
Example: isPrefix([2,4], [ ]) = false.
isPrefix(A, B) = false when head(A) != head(B)
Example: isPrefix([3,4,6], [2,4,6,8,10]) = false.
isPrefix(A, B) = isPrefix(tail(A), tail(B)) (when head(A) = head(B))
Example: isPrefix([2,4,6], [2,4,6,8,10]) = isPrefix([4,6], [4,6,8,10]) = true.