go1 :- initialize(count),member(2, [1,2,3]), get_value(count,X), printf("Count is: %\n", [X]).

/* go2 assumes count has been initialized */
go2 :- member(a, [b, c, d]).

/* try this:
go1.
go2.
get_value(count, X).
*/

member(Item, [Item|Tail]) :- increment(count).
member(Item, [Head|Tail]) :- increment(count), member(Item, Tail).

/*
 Create the von Neumann variable 'Var' and initialize it to 0
 Notes:
 The 'dynamic' statement is required to allow for retraction.
 'initialize' creates a new rule of the form:
 von_Nuemann_store(Var, 0) :- true.
 The reason we use this form, rather than simply 'von_Nuemann_store(Var,0).'
 is that in 'increment' and 'set_value', we use the 'rule' predicate to search
 for the current rule defining the value of Var, and the 'rule' predicate
 requires a head and a body.
*/
initialize(Var) :- dynamic(von_Nuemann_store,2),
		retractall(von_Nuemann_store(Var,Value)),
		assert(von_Nuemann_store(Var, 0):-true).

/* add one to von Neumann variable 'Var' */
increment(Var) :- rule(von_Nuemann_store(Var, Value),true),
                  retract(von_Nuemann_store(Var, Value):-true),
                  NewValue = Value+1,
                  assert(von_Nuemann_store(Var, NewValue):-true).

/* set value of von Neumann variable 'Var' to 'NewValue' */
set_value(Var, NewValue) :- rule(von_Nuemann_store(Var, Value),true),
                  retract(von_Nuemann_store(Var, Value):-true),
                  assert(von_Nuemann_store(Var, NewValue):-true).

/* get value of von Neumann variable 'Var' returning it in 'Value' */
get_value(Var, Value) :- von_Nuemann_store(Var, Value).