(stack = "aaaaaaa
bbbbbbb
123 nay!
ccccccc
123 yeah!
ddddddd
yes yes
yes no
eeeeeee")
(target = "yes")
(previousLines = (CircularBuffer . new(2)))
(stack . each { |line|
   (line . chomp!)
   (if (line . include? target)
      (break)
    end)
   (previousLines . enter line)})
(printf "two lines before the target line is '%s'\n" , (previousLines . nth 1))



Here is for example a CircularBuffer that would do:

(class Node
   (attr_accessor :data,:previous,:next)
   (def initialize(data)
      (@data = data)
      (@previous = nil)
      (@next = nil)
    end)
end)

(class CircularBuffer
   (def initialize(maxSize)
      (@maxSize = maxSize)
      (@size = 0)
      (@tail = nil)
    end)
   (def enter(data)
      (if (@size == @maxSize)
         (@tail . data= data)
         (@tail = (@tail . next))
       elsif (@size == 0)
         (@tail = (Node . new data))
         (@tail . previous= @tail)
         (@tail . next=     @tail)
         (@size = 1)
       else
         (node = (Node . new data))
         (node . previous= (@tail . previous))
         (node . next=     @tail)
         (@tail . previous . next = node)
         (@tail . previous = node)
         (@size = (@size + 1))
       end)
    end)
   (def nth(index)
      (index = (index . modulo @maxSize))
      (cur = (@tail . previous))
      (while (0 < index)
         (cur = (cur . previous))
         (index = (index - 1))
       end)
       (cur . data)
    end)
end)