CSE526: Principles of Programming Languages
			      Scott Stoller
	hw5: shared-variable concurrency, lambda calculus
			version: 5pm,5mar2004
			   due: 11mar2004

1. Consider adding a new command CU ("conditional update") to the
programming language of chapter 8.  The syntax of CU is:

 <comm> ::= CU(<var>,<var>,<intexp>,<var>)

Informally, the semantics of CU(x,x0,e,flag) is that it
executes the following two atomic steps.

 (1) evaluate the expression e to an integer i

 (2) if x=x0 then (x:=i; flag:=1) else flag:=0

Each of these two steps executes atomically.  Transitions of other
threads may occur between the steps.

Extend the transition semantics of section 8.1 with transition
rule(s) for CU.  Hint: One issue is where to store the value i
between the two steps.  One possible approach is to augment the 
variety of configurations by introducing an additional kind of 
command.


2. Exercise 10.1 (a), (b), (e) for the second of the three expressions.
You don't need to do parts (c) and (d).  You don't need to do anything 
for the first and third expressions.