doc-src/Intro/list.thy

* moved ThreeDivides from Isar_examples to better suited HOL/ex
* moved 2 summation lemmas from ThreeDivides to SetInterval

List = FOL +
types 	list 1
arities list	:: (term)term
consts	Nil	:: "'a list"
   	Cons	:: "['a, 'a list] => 'a list" 
end