Require it to be scalar.

chapters/functions.tex

@@ -1631,7 +1631,7 @@ \subsection{Partial Derivatives of Functions}\label{partial-derivatives-of-funct
 \end{lstlisting}
 is the partial derivative of a function, and may only be used as declarations of functions.
 
-The semantics is that a function (and only a function) can be specified in this form, defining that it is the partial derivative of the function to the right of the equal sign (looked up in the same way as a short class definition, and the looked up name must be a function), and partially differentiated with respect to each {\lstinline!IDENT!} in order (starting from the first one).  The {\lstinline!IDENT!} must be {\lstinline!Real!} inputs to the function.
+The semantics is that a function (and only a function) can be specified in this form, defining that it is the partial derivative of the function to the right of the equal sign (looked up in the same way as a short class definition, and the looked up name must be a function), and partially differentiated with respect to each {\lstinline!IDENT!} in order (starting from the first one).  The {\lstinline!IDENT!} must be scalar {\lstinline!Real!} inputs to the function.
 
 The comment allows a user to comment the function (in the info-layer and as one-line description, and as icon).