src/main/java/com/bazaarvoice/jless/parser/Parser.java
Rule Value() {
        return FirstOf(
                Keyword(),
                Literal(),
                Function(),
                VariableReference(),
                URL(),
                Font(),
                AlphaFilter(),
                ExpressionFunction(),
                FilterFunction()
        );
    }
Rule Value() {
        return FirstOf(
                Keyword(),
                Literal(),
                Function(),
                VariableReference(),
                URL(),
                Font(),
                AlphaFilter(),
                ExpressionFunction(),
                FilterFunction()
        );
    }
Any token used as a value in an expression
Future: Accessors