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16
4a1s/MFES/PL - Aula 4.md
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@ -26,20 +26,30 @@ sig Day {}
pred inv1 { pred inv1 {
// Every image is posted by one user. // Every image is posted by one user.
all y: Photo | some x: User | x->y in posts all y: Photo | some x: User | x->y in posts
all x,y : User, z: Photo | x->z in posta all x,y : User, z: Photo | x->z in posts and y->z in posts implies x=y
// or (equivalent to 2nd formula)
all x,y : User, z: Photo | x in posts z and y in posts z implies x=y
// or (equivalent to 1st and 2nd formula)
all y: Photo | some posts y
all y: Photo | lone posts y
// or (equivalent to 1st and 2nd formula)
all y:Photo | one posts y
} }
pred inv2 { pred inv2 {
// Users cannot follow themselves. // Users cannot follow themselves.
all x: User | x->x not in follows
} }
pred inv3 { pred inv3 {
// Users can see ads posted by everyone, // Users can see ads posted by everyone,
// but only see non ads posted by followed users. // but only see non ads posted by followed users.
} }