@Nonnull
    private Rule rule( FormattableContainer parent, String selector, Operation params, Expression guard ) {
        Rule rule = new Rule( reader, parent, selector, params, guard );
        parseRule( rule );
        return rule;
    }
@ Nonnull private Rule rule ( FormattableContainer parent , String selector , Operation params , Expression guard ) { Rule rule = new Rule ( reader , parent , selector , params , guard ) ; parseRule ( rule ) ; return rule ; }
Create a rule and parse the content of an block.

@param selector the selectors
@param parent the parent in the hierarchy
@param params the parameters if it is a mixin.
@param guard an optional guard expression
@return the rule