code: public void importStylesheet(Stylesheet sheet) {
          if (sheet == null) {
              return;
          }
          if (importedSheets.contains(sheet.getName())) {
              return;
          }
          importedSheets.add(sheet.getName());
          for (String imp : sheet.getImports()) {
              importStylesheet(imp);
          }
          for (Mixin mix : sheet.getMixins()) {
              mixins.put(mix.getName(), mix);
          }
          for (Variable var : sheet.getVariables()) {
              if (!scope.has(var.getName()) || !var.isDefaultValue()) {
                  scope.set(var.getName(), var.getValue());
              } else {
                  debug("Skipping redundant variable definition: '" + var + "'");
              }
          }
          for (Section section : sheet.getSections()) {
              List<Section> stack = new ArrayList<>();
              expand(null, section, stack);
          }
      }
docs: Imports an already parsed stylesheet.
  
  @param sheet the stylesheet to import