| A precise statement of the classification axiom scheme
| requires a description of formulae.  It is agreed that: †
|
| a.  The result of replacing &quot;!a!&quot; and &quot;!b!&quot;
|     by variables is, for each of the following,
|     a formula.
|
|     !a! = !b!
|
|     !a! in !b!
|
| b.  The result of replacing &quot;!a!&quot; and &quot;!b!&quot; by variables
|     and &quot;A&quot; and &quot;B&quot; by formulae is, for each of the following,
|     a formula.
|
|     if A, then B
|
|     A iff B
|