1. isPrefix([ ], L) = true

    Example: isPrefix([ ], [2,4,6]) = true.

  2. isPrefix(A, [ ]) = false

    Example: isPrefix([2,4], [ ]) = false.

  3. isPrefix(A, B) = false   when head(A) != head(B)

    Example: isPrefix([3,4,6], [2,4,6,8,10]) = false.

  4. 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.