Closes #2232

Note that this required a bit more rewriting since replacing ISO-standard by SI-standard from BIPM.
I have not seen the ISO-standard so I don't know if the previous text was consistent with it or not.

chapters/unitexpressions.tex

@@ -1,13 +1,17 @@
 \chapter{Unit Expressions}\doublelabel{unit-expressions}
 
 Unless otherwise stated, the syntax and semantics of unit expressions in
-Modelica are conform with the international standards ISO 31/0-1992
+Modelica conform with the international standards 
+"International System of Units (SI)" by BIPM superseding parts of 
+ISO 31/0-1992
 "General principles concerning quantities, units and symbols" and ISO
 1000-1992 "SI units and recommendations for the use of their multiples
-and of certain other units". Unfortunately, neither these two standards
-nor other existing or emerging ISO standards define a formal syntax for
+and of certain other units". 
+Unfortunately, these standards do not define a formal syntax for
 unit expressions. There are recommendations and Modelica exploits them.
 
+Note that this document uses the American spelling meter, whereas the SI specification from BIPM uses the British spelling metre.
+
 Examples for the syntax of unit expressions used in Modelica: "\lstinline!N.m!",
 "\lstinline!kg.m/s2!", "\lstinline!kg.m.s-2!" "\lstinline!1/rad!", 
 "\lstinline!mm/s!".
@@ -25,10 +29,10 @@ \section{The Syntax of Unit Expressions}\doublelabel{the-syntax-of-unit-expressi
 \end{lstlisting}
 
 The unit of measure of a dimension free quantity is denoted by "1". The
-ISO standard does not define any precedence between multiplications and
-divisions. The ISO recommendation is to have at most one division, where
+SI standard does not define any precedence between multiplications and
+divisions. The SI recommendation is to have at most one division, where
 the expression to the right of "\lstinline!/!" either contains no multiplications or
-is enclosed within parentheses. It is also possible to use negative
+is enclosed within parentheses, or use negative
 exponents, for example, "\lstinline!J/(kg.K)!" may be written as "\lstinline!J.kg-1.K-1!".
 
 \begin{lstlisting}[language=grammar]
@@ -39,15 +43,13 @@ \section{The Syntax of Unit Expressions}\doublelabel{the-syntax-of-unit-expressi
    "."
 \end{lstlisting}
 
-The ISO standard allows that a multiplication operator symbol is left
-out. However, Modelica enforces the ISO recommendation that each
-multiplication operator is explicitly written out in formal
-specifications. For example, Modelica does not support "\lstinline!Nm!" for
-newtonmeter, but requires it to written as "\lstinline!N.m!".
+The SI standard specifies that a multiplication operator symbol is written as space 
+or as a dot. For example, Modelica does not support "\lstinline!Nm!" for
+newton-meter, 
+but requires it to written as "\lstinline!N.m!". 
 
-The preferred ISO symbol for the multiplication operator is a "dot" a
-bit above the base line: "·". Modelica supports the ISO alternative ".",
-which is an ordinary "dot" on the base line.
+The SI standard requires that this "dot" is a bit above the base line: "·", which is not part of ASCII. 
+The ISO standard also prefers "·", but Modelica supports the ISO alternative ".", which is an ordinary "dot" on the base line.
 
 \begin{lstlisting}[language=grammar]
 unit_factor:
@@ -57,7 +59,7 @@ \section{The Syntax of Unit Expressions}\doublelabel{the-syntax-of-unit-expressi
    [ "+" | "-" ] integer
 \end{lstlisting}
 
-The ISO standard does not define any operator symbol for exponentiation.
+The SI standard uses super-script for the exponentation, and does thus not define any operator symbol for exponentiation.
 A \lstinline!unit_factor! consists of a \lstinline!unit_operand! possibly suffixed by a
 possibly signed integer number, which is interpreted as an exponent.
 There must be no spacing between the \lstinline!unit_operand! and a possible
@@ -92,13 +94,13 @@ \section{Examples}\doublelabel{examples2}
 
 The unit expression "\lstinline!m!" means meter and not milli
 (10\textsuperscript{-3}), since prefixes cannot be used in isolation.
-For millimeter use "\lstinline!mm!" and for squaremeter, m\textsuperscript{2}, write
+For millimeter use "\lstinline!mm!" and for square meter, m\textsuperscript{2}, write
 "\lstinline!m2!".
 
 The expression "\lstinline!mm2!" means \lstinline!mm2! = (10\textsuperscript{-3}m)2 =
 10\textsuperscript{-6}m\textsuperscript{2}. Note that exponentiation
 includes the prefix.
 
-The unit expression "\lstinline!T!" means Tesla, but note that the letter "\lstinline!T!" is
+The unit expression "\lstinline!T!" means tesla, but note that the letter "\lstinline!T!" is
 also the symbol for the prefix \lstinline!tera! which has a multiplier value of
 10\textsuperscript{12}.