adapted example grammar for expressions

.../org/xtext/example/mydsl/myDsl/impl/MyDslFactoryImpl.java

.../org/xtext/example/mydsl/myDsl/impl/MyDslPackageImpl.java

...src-gen/org/xtext/example/mydsl/myDsl/impl/PowerImpl.java

...c-gen/org/xtext/example/mydsl/myDsl/impl/ProductImpl.java

.../src-gen/org/xtext/example/mydsl/myDsl/impl/TermImpl.java

.../example/mydsl/myDsl/impl/UnaryBooleanExpressionImpl.java

...g/xtext/example/mydsl/myDsl/util/MyDslAdapterFactory.java

...c-gen/org/xtext/example/mydsl/myDsl/util/MyDslSwitch.java

...xtext/example/mydsl/parser/antlr/internal/InternalMyDsl.g

.../example/mydsl/parser/antlr/internal/InternalMyDsl.tokens

...ample/mydsl/parser/antlr/internal/InternalMyDslLexer.java

...mple/mydsl/parser/antlr/internal/InternalMyDslParser.java

...text/example/mydsl/serializer/MyDslSemanticSequencer.java

...ext/example/mydsl/serializer/MyDslSyntacticSequencer.java

.../org/xtext/example/mydsl/services/MyDslGrammarAccess.java

...ext.example.mydsl/src/org/xtext/example/mydsl/MyDsl.xtext