prefix([],x)      = true
    prefix(h::t,[])   = false
    prefix(a::u,b::v) = (a == b) and prefix(u,v)

     prefix([1,2], [1,2,3,4])
        = (1 == 1) and prefix([2], [2,3,4])
        = true and prefix([2], [2,3,4])
        = true and ((2 == 2) and prefix([], [3,4]))
        = true and (true and (prefix([], [3,4])))
        = true and (true and true)
        = true and true
        = true

Note: This function uses the boolean 'and' operator, which is often implemented as a conditional expression: A and B is the same as cond(A, B, false) (or, written in Cinnameg, If A then B else false %If). The significance of this is that evaluation of 'and' expressions is not done inside-out. If a conditional 'and' expression is used, then evaluation of prefix([1,2], [1,2,3,4]) is as follows.

     prefix([1,2], [1,2,3,4])
        = (1 == 1) and prefix([2], [2,3,4])
        = true and prefix([2], [2,3,4])
        = prefix([2], [2,3,4])
        = (2 == 2) and prefix([], [3,4])
        = true and prefix([], [3,4])
        = prefix([], [3,4])
        = true