code: @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;
      }
docs: 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