code: function removeUnusedVariableDeclarators (ast) {
    var unused = findUnusedVariableDeclarators(ast);
    traverse.traverse(ast, {
      enter: function (node, parent) {
        if (node.type !== 'VariableDeclaration') {
          return;
        }
        node.declarations = fast.filter(node.declarations, function (item) {
          return !~fast.indexOf(unused, item);
        });
      }
    });
    return ast;
  }
docs: If a variable is declared, but not used, remove it.