Support non-integer exponents to handle the result of square.

chapters/unitexpressions.tex

@@ -43,10 +43,14 @@ \section{The Syntax of Unit Expressions}\label{the-syntax-of-unit-expressions}
 
 unit-exponent :
    [ "+" | "-" ] UNSIGNED-INTEGER
+   | "^" "(" [ "+" | "-" ] UNSIGNED-INTEGER "/" UNSIGNED-INTEGER ")"
 \end{lstlisting}
 
 The SI standard uses super-script for the exponentation, and does thus not define any operator symbol for exponentiation.
-A \lstinline[language=grammar]!unit-factor! consists of a \lstinline[language=grammar]!unit-operand! possibly suffixed by a possibly signed integer number, which is interpreted as an exponent.
+A \lstinline[language=grammar]!unit-factor! normally consists of a \lstinline[language=grammar]!unit-operand! possibly suffixed by a possibly signed integer number, which is interpreted as an exponent.
+Non-integer exponents are needed to fully support the unit for square roots (and can be used for roots in general).
+They are intended to be only be used when needed, and thus the fraction must be simplified and the denominator may not be one (or zero).
+They use an explicit exponentation operator to be easy to recognize.
 There must be no spacing between the \lstinline[language=grammar]!unit-operand! and a possible \lstinline[language=grammar]!unit-exponent!.
 
 \begin{lstlisting}[language=grammar]