diff --git a/book/book_second_edition.pdf b/book/book_second_edition.pdf
index c8dee89..4495060 100644
Binary files a/book/book_second_edition.pdf and b/book/book_second_edition.pdf differ
diff --git a/src/abstract_structures/__init__.py b/src/abstract_structures/__init__.py
new file mode 100644
index 0000000..be4c726
--- /dev/null
+++ b/src/abstract_structures/__init__.py
@@ -0,0 +1 @@
+__all__=["hash_tables", "heap", "linked_list", "queues", "stacks"]
\ No newline at end of file
diff --git a/src/abstract_structures/hash_table.py b/src/abstract_structures/hash_table.py
deleted file mode 100644
index e367ea7..0000000
--- a/src/abstract_structures/hash_table.py
+++ /dev/null
@@ -1,65 +0,0 @@
-#!/usr/bin/python3
-# mari von steinkirch @2013
-# steinkirch at gmail
-
-''' The first example design a Hash table using chaining to avoid collisions. The second 
-    uses two lists.'''
-
-from linked_list_fifo import Node, LinkList
-
-class HashTableLL(object):
-    def __init__(self, size):
-        self.size = size
-        self.slots = []
-        self._createHashTable()
-
-    def _createHashTable(self):
-        for i in range(self.size) :
-            self.slots.append(LinkList())
-    
-    def findIndex(self, item):
-        return item%self.size
-
-    def addItem(self, item):
-        index = self.findIndex(item)
-        self.slots[index].addNode(item)
-        return self
-
-    
-    def delItem(self, item):
-        index = self.findIndex(item)
-        self.slots[index].deleteNode(item)
-        return self
-
-
-    def printHashTable(self):
-        for i in range(self.size):
-            print('\nSlot {}:'.format(i))
-            print(self.slots[i].printList())
-            
-    
-
-       
-
-def test_hash_tables():
-    H1 = HashTableLL(3)
-    for i in range (0, 20):
-        H1.addItem(i)
-    H1.printHashTable()
-    print('\n\nNow deleting:')
-    H1.delItem(0)   
-    H1.delItem(0)        
-    H1.delItem(0) 
-    H1.delItem(0) 
-    H1.printHashTable()
-'''
-    H2 = HashTable2L(3)
-    H2[54]="buffy"
-    H2[26]="xander"
-    H2[17]="giles"
-    print(H.2slots)
-'''
-
-
-if __name__ == '__main__':
-    test_hash_tables()       
diff --git a/src/abstract_structures/hash_tables/__init__.py b/src/abstract_structures/hash_tables/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/src/abstract_structures/hash_tables/hash_table.py b/src/abstract_structures/hash_tables/hash_table.py
new file mode 100644
index 0000000..61c41a4
--- /dev/null
+++ b/src/abstract_structures/hash_tables/hash_table.py
@@ -0,0 +1,61 @@
+#!/usr/bin/python
+
+__author__ = "Mari Wahl"
+__email__ = "marina.w4hl@gmail.com"
+
+''' design a Hash table using chaining to avoid collisions.'''
+
+
+#import abstract_structures.linked_list.linked_list_fifo
+#import abstract_structures.linked_list.node
+
+from linked_list_fifo import LinkedListFIFO
+
+
+
+class HashTableLL(object):
+    def __init__(self, size):
+        self.size = size
+        self.slots = []
+        self._createHashTable()
+
+    def _createHashTable(self):
+        for i in range(self.size) :
+            self.slots.append(LinkedListFIFO())
+
+    def _find(self, item):
+        return item%self.size
+
+    def _add(self, item):
+        index = self._find(item)
+        self.slots[index].addNode(item)
+
+    def _delete(self, item):
+        index = self._find(item)
+        self.slots[index].deleteNode(item)
+
+    def _print(self):
+        for i in range(self.size):
+            print('\nSlot {}:'.format(i))
+            print(self.slots[i]._printList())
+
+
+
+
+def test_hash_tables():
+    H1 = HashTableLL(3)
+    for i in range (0, 20):
+        H1._add(i)
+    H1._print()
+    print('\n\nNow deleting:')
+    H1._delete(0)
+    H1._delete(1)
+    H1._delete(2)
+    H1._delete(0)
+
+    H1._print()
+
+
+
+if __name__ == '__main__':
+    test_hash_tables()
diff --git a/src/abstract_structures/linked_list/linked_list_fifo.py b/src/abstract_structures/linked_list/linked_list_fifo.py
index 8a75751..1492795 100644
--- a/src/abstract_structures/linked_list/linked_list_fifo.py
+++ b/src/abstract_structures/linked_list/linked_list_fifo.py
@@ -50,12 +50,6 @@ class LinkedListFIFO(object):
             self.tail.pointer = node
         self.tail = node
 
-    # delete a node in some position
-    def _delete(self, prev, node):
-        self.length -= 1
-        prev.pointer = node.pointer
-        if not node.pointer:
-            self.tail = prev
 
     # add nodes in general
     def addNode(self, value):
@@ -81,8 +75,14 @@ class LinkedListFIFO(object):
             self._deleteFirst()
         else:
             node, prev, i = self._find(index)
-            if i == index:
-                self._delete(prev, node)
+            if i == index and node:
+                self.length -= 1
+                if i == 0 or not prev :
+                    self.head = node.pointer
+                else:
+                    prev.pointer = node.pointer
+                if not self.tail == node:
+                    self.tail = prev
             else:
                 print('Node with index {} not found'.format(index))
 
diff --git a/src/abstract_structures/linked_list/linked_list_from_stack.py b/src/abstract_structures/linked_list/linked_list_from_stack.py
deleted file mode 100644
index 52efaf6..0000000
--- a/src/abstract_structures/linked_list/linked_list_from_stack.py
+++ /dev/null
@@ -1,71 +0,0 @@
-#!/usr/bin/python3
-# mari von steinkirch @2013
-# steinkirch at gmail
-
-
-
-class Node(object):
-	def __init__(self,data=None,next=None):
-		self.data = data
-		self.next = next
-		
-	def setnext(self,next):
-		self.next = next
-		
-	def __str__(self):
-		return "%s" % self.data
-
-
-class LinkedListStack(object):
-    ''' linked list from stack '''
-    def __init__(self, max=0):
-        self.max = max
-        self.head = None
-        self.z = None
-        self.size = 0
-        
-    def push(self, data):
-        if self.size == 0:
-            self.head = Node(data=data)
-            self.size += 1
-        else:
-            head = self.head
-            temp = Node(data = data)
-            self.head = temp
-            self.head = temp
-            temp.setnext(head)
-
-    def pop(self):
-        temp = self.head.next
-        self.head = temp
-        
-    def isEmpty(self):
-        return self.size == 0
-            
-    def __str__(self):
-        d = ""
-        if self.isEmpty(): return ""
-        else:
-            temp = self.head
-            d += "%s\n" % temp
-            while temp.next != None:
-                temp = temp.next
-                d += "%s\n" % temp
-            return d
-   
-				
-					
-    
-def test_ll_from_stack():
-	ll = LinkedListStack(max = 20)
-	ll.push("1")
-	ll.push("2")
-	ll.push("3")
-	ll.push("4")
-	print(ll)
-	ll.pop()
-	print(ll)
-	
-
-if __name__ == '__main__':
-    test_ll_from_stack()
diff --git a/src/abstract_structures/queues_stacks/queue_2_stacks.py b/src/abstract_structures/queues/queue_2_stacks.py
similarity index 100%
rename from src/abstract_structures/queues_stacks/queue_2_stacks.py
rename to src/abstract_structures/queues/queue_2_stacks.py
diff --git a/src/abstract_structures/queues_stacks/animal_shelter.py b/src/abstract_structures/stacks/queues_stacks/animal_shelter.py
similarity index 100%
rename from src/abstract_structures/queues_stacks/animal_shelter.py
rename to src/abstract_structures/stacks/queues_stacks/animal_shelter.py
diff --git a/src/abstract_structures/queues_stacks/setofStacks.py b/src/abstract_structures/stacks/queues_stacks/setofStacks.py
similarity index 100%
rename from src/abstract_structures/queues_stacks/setofStacks.py
rename to src/abstract_structures/stacks/queues_stacks/setofStacks.py
diff --git a/src/abstract_structures/queues_stacks/setofStacksList.py b/src/abstract_structures/stacks/queues_stacks/setofStacksList.py
similarity index 100%
rename from src/abstract_structures/queues_stacks/setofStacksList.py
rename to src/abstract_structures/stacks/queues_stacks/setofStacksList.py
diff --git a/src/abstract_structures/queues_stacks/sort_stack.py b/src/abstract_structures/stacks/queues_stacks/sort_stack.py
similarity index 100%
rename from src/abstract_structures/queues_stacks/sort_stack.py
rename to src/abstract_structures/stacks/queues_stacks/sort_stack.py
diff --git a/src/abstract_structures/queues_stacks/stackMinElemO1.py b/src/abstract_structures/stacks/queues_stacks/stackMinElemO1.py
similarity index 100%
rename from src/abstract_structures/queues_stacks/stackMinElemO1.py
rename to src/abstract_structures/stacks/queues_stacks/stackMinElemO1.py
diff --git a/src/abstract_structures/queues_stacks/stack_.py b/src/abstract_structures/stacks/queues_stacks/stack_.py
similarity index 100%
rename from src/abstract_structures/queues_stacks/stack_.py
rename to src/abstract_structures/stacks/queues_stacks/stack_.py
diff --git a/src/abstract_structures/queues_stacks/towers_of_hanoi.py b/src/abstract_structures/stacks/queues_stacks/towers_of_hanoi.py
similarity index 100%
rename from src/abstract_structures/queues_stacks/towers_of_hanoi.py
rename to src/abstract_structures/stacks/queues_stacks/towers_of_hanoi.py