// field ::= 'fst' | 'snd'

// Relations representing the program:
// ploadconst : loc -> reg -> num -> type  (l : x = n)
// pallocpair : loc -> reg -> reg -> reg -> type (l : x = < y , z >)
// pgetfield  : loc -> reg -> reg -> field -> type (l : x = y.f) 
// psetfield  : loc -> reg -> field -> reg -> type (l : x.f = y) 

// values are numbers or locations:
// value = <num> | loc(l)

// The relations we want to define:
// valueofreg   : reg -> value -> type
// valueoffield : loc -> field -> value -> type


// TASK:

// Rule 1: If ploadconst(?l, ?r, ?n),
//         what can you say about the value of register ?r ?

// Rule 2: If pgetfield(?l2, ?r1, ?r2, ?f)
//         and register ?r2 can have value ?l
//         and field ?l.?f can have value ?v,
//         what can you say about the value of r1?

// Rule 3: If 	psetfield(?l2 , ?r1 , ?f , ?r2),
//         what do you need to know about the values of ?r1 and ?r2
//         to know something about the value of the field ?l.?f ?

// Rules 4 5 and 6: If 	pallocpair(?l , ?r , ?r1 , ?r2),
//                  what can you say about
//                  the values of:
//                  (a) the field ?l.'fst'
//                  (b) the field ?l.'snd'
//                  (c) the register ?r


// ----------------------------------------------------------------------

// the progam

ploadconst('l1', 'w', 0).  // l1 : w = 0
ploadconst('l2', 'x', 1).  // l2 : x = 1
ploadconst('l3', 'y', 2).  // l3 : y = 2
ploadconst('l4', 'z', 3).  // l4 : z = 3
pallocpair('l5', 'p1', 'w', 'x'). // l5 : p1 = < w , x >
pallocpair('l6', 'p2', 'y', 'z'). // l6 : p2 = < y , z >
pallocpair('l7' , 'p' , 'p1' , 'p2'). // l7 : p = <p1, p2>
psetfield('l12', 'p', 'fst', 'p2'). // l12 : p.fst = p2
psetfield('l13', 'p', 'snd', 'p1'). // l13 : p.snd = p1
pgetfield('l8', 'p1' , 'p'  , 'fst'). // l8 : p1 = p.fst
pgetfield('l9', 'p2' , 'p'  , 'snd'). // l9 : p2 = p.snd
pgetfield('l10', 'r1', 'p1' , 'fst'). // l10 : r1 = p1.fst
pgetfield('l11', 'r2', 'p2' , 'snd'). // l11 : r2 = p2.snd

// SHOULD BE: 0,2
?- valueofreg('r1',?v1).
// SHOULD BE: 3,1
?- valueofreg('r2',?v2).