Example

If we replace $x$ by $\square$ wherever $x$ occurs in the assembly $\vee\in xy=xx$, we obtain the assembly $\vee\in\square y=\square\square$.

If $A$ is an assembly and we are interested particularly in a letter $x$, or two distinct letters $x$ and $y$ (which may or may not appear in $A$), we shall often write $A\{x\}$ or $A\{x,y\}$. In this case we write $A\{B\}$ instead of $(B\mathbin{|}x)\,A$. We denote by $A\{B,C\}$ the assembly obtained by simultaneously replacing $x$ by $B$ and $y$ by $C$ wherever they occur in $A$ (note that $x$ and $y$ may appear in $B$ and in $C$); if $x'$ and $y'$ are distinct letters, other than $x$ and $y$, which appear in neither $A$, $B$, nor $C$, then $A\{B,C\}$ is the same as $(B\mathbin{|}x')(C\mathbin{|}y')(x'\mathbin{|}x)(y'\mathbin{|}y)A$.