* the SASL interpreter
** thunks = expression + environment
*** printing thunks
*** evaluation

* real SASL
** if syntax
** pattern matching (as in ML)
** ZF expressions

* lambda calculus
** key idea: using thunks to represent data and control
** booleans and if, p. 179
** lists, p. 179
** the calculus, p. 187
*** substitution
****	free variables, Fv
	----------
	Fv(x) = {x}
	Fv(lambda (x) e) = Fv(e) - {x}
	Fv(e e') = Fv(e) union Fv(e')
	----------
****	bound variables
	------------------
	[alpha] (lambda (x) U) = (lambda (y) [y/x]U), if y is free in U
	------------------
****	syntactic substitution
	------------
	[e/x]x         == e
	[e/x]y         == y		    if not(y == x)
	[e/x](e1 e2)   == (([e/x]e1) ([e/x]e2))
	[e/x](lambda (y) e') == (lambda (y) [e/x]e') if not(y == x),
							and not(y in Fv(e))
	------------
*** equational theory
****	beta rule: application
	----------------------
	[beta] ((lambda (x) M) N) ==> [N/x]M
	------------------
	------------------
	e.g.:	((((lambda (f) (lambda (x) (lambda (y) (f x y)))) +) 1) 2)
		==> ((((lambda (x) (lambda (y) (+ x y)))) 1) 2)
		==> ((lambda (y) (+ 1 y)) 2)
		==> (+ 1 2)
	------------------
		------------
		(((lambda (y) (lambda (x) y)) 27) (/ 3 0))
		==> ((lambda (x) 27) (/ 3 0))
		==>  27
		------------
****	delta rule: using primitive operations from the environment
	---------------------
	[delta] (f e1 e2) ==> f(e1,e2), if e1 and e2 are integers,
					and f is not bound by surrounding expr.
	---------------------
****	mu rule: using defined names from the environment
	---------------------
	[delta] f ==> e, if f is not bound by surrounding expr,
			 and f is bound to e in global environment
	---------------------
****	eta rule: an optimization
	---------------
	[eta]	(lambda (x) (e x)) ==> e, if x not in Fv(e)
	---------------
	---------------
	e.g.: (set with-characteristic (lambda (f) (lambda (x) (f x))))
		==> (set with-characteristic (lambda (f) f))
	e.g.: (set member? (lambda (s) (lambda (x) (s x))))
		==> (set member? (lambda (s) s))
	---------------
** summary
***	referential transparency: equals can be replace by equals
***	why give up referential transparency?