Write error condition in parenthesis instead of at start of sentence

Addressing review comment by Hans.

chapters/functions.tex

@@ -444,8 +444,8 @@ \subsection{Positional or Named Input Arguments of Functions}\doublelabel{positi
 order of the parameters is given by the order of the component
 declarations in the function definition. Next, for each named argument
 \lstinline!identifier = expression!, the \lstinline!identifier! is used to determine the
-corresponding slot. It is an error if this slot is already filled, and the value of the argument is placed in the slot,
-filling it. When all arguments have been processed, the slots that are
+corresponding slot.  The value of the argument is placed in the slot,
+filling it (it is an error if this slot is already filled).  When all arguments have been processed, the slots that are
 still unfilled are filled with the corresponding default value of the
 function definition. The default values may depend on other inputs
 (these dependencies must be acyclical in the function) -- the values for