what is the fourth property for a frame?

[drvdw]

In the section on frames it says:

"Note that $d statements are not part of the set of mandatory hypotheses,
and their order doesn’t matter (as long as they satisfy the fourth property for
a frame described above)"

but what I see is this list

"A frame has the following properties:
\begin{enumerate}
\item The set of variables contained in its \texttt{$f} statements must
be identical to the set of variables contained in its \texttt{$e},
\texttt{$a}, and/or \texttt{$p} statements. In other words, each
variable in a \texttt{$e}, \texttt{$a}, or \texttt{$p} statement must
have an associated ``variable type'' defined for it in a \texttt{$f}
statement.
\item No two \texttt{$f} statements may contain the same variable.
\item Any \texttt{$f} statement
must occur before a \texttt{$e} statement in which its variable occurs.
\end{enumerate}"

but it has just 3 properties. What is the forth property?

[benjub]

The first sentence you quote should probably be something like

... (as long as they all occur before the (final and unique) $a or $p statement of the frame).

Although this is not needed, it would probably be cleaner to require that $d statements only involve mandatory variables (or, for extended frames, dummy variables as well).