src/main/java/com/bazaarvoice/jless/parser/Parser.java
Rule RuleSet() {
        return Sequence(
                FirstOf(
                        // Standard CSS rule set
                        Sequence(
                                SelectorGroup(), push(new RuleSetNode(pop())),
                                '{',
                                Scope(), peek(1).addChild(pop()), Ws0()
                        ),
                        // Mixin rule set, with possible arguments
                        Sequence(
                                ClassSelectorGroup(), push(new RuleSetNode(pop())), Ws0(),
                                Parameters(), Ws0(),
                                '{',
                                Scope(), peek(1).addChild(pop()), peek(1).addChild(pop()), Ws0()
                        )
                ),
                '}', Ws0Nodes()
        );
    }
Rule RuleSet() {
        return Sequence(
                FirstOf(
                        // Standard CSS rule set
                        Sequence(
                                SelectorGroup(), push(new RuleSetNode(pop())),
                                '{',
                                Scope(), peek(1).addChild(pop()), Ws0()
                        ),
                        // Mixin rule set, with possible arguments
                        Sequence(
                                ClassSelectorGroup(), push(new RuleSetNode(pop())), Ws0(),
                                Parameters(), Ws0(),
                                '{',
                                Scope(), peek(1).addChild(pop()), peek(1).addChild(pop()), Ws0()
                        )
                ),
                '}', Ws0Nodes()
        );
    }
(Selectors '{' Ws0 / Class Ws0 Parameters Ws0 '{' Ws0) Scope Ws0 '}' Ws0

Ex: div, .class, body > p {...}

Future: Hidden