Also describe unit mappings of transcendental functions to match othe…

…r proposal

chapters/unitexpressions.tex

@@ -262,7 +262,7 @@ \subsection{Expressions with Empty Unit}\label{expressions-with-empty-unit}
   \lstinline!Integer! expressions implicitly cast to \lstinline!Real!.
 \end{itemize}
 
-Expressions defined to \emph{not} propagate the empty unit, thereby forcing inference of unit \lstinline!"1"!:
+Expressions defined to \emph{not} propagate the empty unit up the expression tree, thereby forcing inference of unit \lstinline!"1"!:
 \begin{itemize}
 \item
   Addition, subtraction, multiplication and division operators when either operand has non-empty unit.
@@ -273,10 +273,22 @@ \subsection{Expressions with Empty Unit}\label{expressions-with-empty-unit}
   (Unit checking involving user-defined functions with empty unit on inputs and outputs is currently not defined.)
 \end{itemize}
 
-Built-in non-array operators, functions, and special expressions that propagate any unit (including empty):
+Built-in non-array operators, functions, and special expressions that propagate any unit (including empty) up the expression tree:
 \begin{itemize}
 \item
-  When all operands have the same unit: negation, addition, and subtraction (scalar or element-wise).
+  When all operands have the same unit: negation, addition, and subtraction (scalar or element-wise), see {array-element-wise-addition-subtraction-and-string-concatenation}.
+\item
+  The \lstinline!abs! function, see \cref{modelica:abs}.
+\end{itemize}
+
+Transcendental functions that propagate both unit \lstinline!"1"! and the empty unit up the expression tree:
+\begin{itemize}
+\item
+  All of the elementary mathematical functions listed in \cref{built-in-mathematical-functions-and-external-built-in-functions}.
+  (Whether some of these also accept other dimensionless units such as \lstinline!"rad"! for the input argument is currently not defined.)
+\item
+  The only binary of these functions, \lstinline!atan2!, requires both arguments to have the same unit, and accepts any unit.
+  The result of \lstinline!atan2! has empty unit only when both arguments have empty unit; otherwise the unit of the result is \lstinline!"1"!.
 \end{itemize}
 
 Special situations in which the empty unit is propagated up the expression tree: