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$$(2+3)+4=5+4=9=2+7=2+(3+4),$$  so we can write also $9=2+3+4$, omitting the parentheses. Actually, this holds for any three numbers in $\ZZ$.  \\  The same property also We claim that this  holds \emph{also}  for the sum of bits. For example $$ (1+0)+1=1+1=0=1+(0+1)=1+0+1.$$ (1+0)+1=1+1=0=1+(0+1)=1+0+1 \,.$$  The reader can verify that it is true our claim  for each any  $a,b,c \in \Fb$.\\ This property can be formalized by saying that both the sum in $\ZZ$ and the sum in $\Fb$ are \emph{associative}. In formulas, we say that $\forall a,b,c$ (which are elements of $\ZZ$ or of $\Fb$), we have  $$(a+b)+c=a+(b+c)=a+b+c.$$  \smallskip