Top/Equations/Answer-27-6

Answer to Question 27-6

Here are the equations for reference. 
(isPrefix.1) isPrefix([], B)        = true
(isPrefix.2) isPrefix(A, [])        = false    (when A is not [])
(isPrefix.3) isPrefix(A, B)  = head(A) == head(B) and 
                               isPrefix(tail(A), tail(B))
                                (when A ≠ [] and B ≠ [])
Evaluation is as follows. 
  isPrefix([2,3], [2])
    = 2 == 2  and  isPrefix([3], [])    by (isPrefix.3)
    = true  and  isPrefix([3], [])
    = isPrefix([3], [])
    = isPrefix(3:[], [])
    = false                             by (isPrefix.2)