Rephrase "base version"

chapters/unitexpressions.tex

@@ -63,9 +63,8 @@ \section{The Syntax of Unit Expressions}\label{the-syntax-of-unit-expressions}
 
 A \lstinline[language=grammar]!unit-symbol! is a non-empty string of \lstinline[language=grammar]!unit-char! characters.
 The set of allowed characters is currently not defined.
-A basic support of units in Modelica should know the basic and derived units of the SI system.
-It is possible to support user defined unit symbols.
-In the base version Greek letters is not supported, but full names must then be written, for example \lstinline!"Ohm"!.
+It is required that basic and derived units of the SI system are recognized, but tools are allowed to additionally support user-defined unit symbols.
+The required unit symbols do not make use of Greek letters, but a unit such as $\Omega$ is spelled out as \lstinline!"Ohm"!.
 
 A \lstinline[language=grammar]!unit-operand! should first be interpreted as a \lstinline[language=grammar]!unit-symbol! and only if not successful the second alternative assuming a prefixed operand should be exploited.
 There must be no spacing between the \lstinline[language=grammar]!unit-symbol! and a possible \lstinline[language=grammar]!unit-prefix!.