Follow set confusion about inclusion The book I am learning about compilers from states that $M \to \alpha N \beta$
will generate a constraint that $\mathrm{FOLLOW}(M)$ is contained in $\mathrm{FOLLOW}(N)$ when $\beta$ is nullable.

This is a bit confusing because there can be other productions such as $M\to \gamma K \theta $.  Logically, then, the set of symbols which can follow $M$  can also be the set of set of symbols that can follow $k$ if $\theta$ is nullable and can be different from $\mathrm{FOLLOW}(N)$.

As such both $\mathrm{FOLLOW}(N)$  and $\mathrm{FOLLOW}(K)$ should be included in $\mathrm{FOLLOW}(M)$. Shouldn't it be like this? 
What might be the reasons behind the statement in the book?
If the grammar includes $M \to \alpha N \beta$ and $\beta$ is nullable then anything which could follow $M$ could also follow $N$. In other words,

$FOLLOW(M) \subset FOLLOW(N)$

So we need to add everything in $FOLLOW(M)$ to $FOLLOW(N)$. It does  not oblige us to alter $M$'s follow set.

If there is also $M \to \gamma K \theta$ with $\theta$ nullable, then similarly we have 

$FOLLOW(M) \subset FOLLOW(K)$ 

so we also have to add everything in $FOLLOW(M)$ to $FOLLOW(K)$. That implies that the FOLLOW sets for $N$ and $K$ are not disjoint, but it really says nothing interesting about $M$'s FOLLOW set.