t;$A$ =&gt; $B$&quot; is not a formula.  For example, if
|    $A$ is &quot;x = y&quot; and $B$ is &quot;y = z&quot;, then '&quot;x = y&quot; =&gt; &quot;y = z&quot;'
|    is not a formula.  Formulae (for example &quot;x = y&quot;) contain no
|    quotation marks.  Instead of &quot;$A$ =&gt; $B$&quot; we want to discuss
|    the result of replacing  &quot;!a!&quot; by $A$  and  &quot;!b!&quot; by $B$  in
|    &quot;!a! =&gt; !b!&quot;.  This sort of circumlocution can be avoided by
|    using Quine's corner 