old_contents: 'use strict';
  
  /* build.js
   * Build CSS from a tree.
   */
  
  const Selector = require('./selector'),
        Property = require('./property'),
        Value = require('./value');
  
  module.exports = exports = build;
  
  function build(tree, indent){
    
  }
  
new_contents: 'use strict';
  
  /* build.js
   * Build CSS from a tree.
   */
  
  const Rule = require('./rule'),
        Property = require('./property'),
        Value = require('./value');
  
  module.exports = exports = render;
  
  function render(tree, opts){
    opts = (typeof opts === 'object') ? opts : [];
  
    if (!Array.isArray(tree))
     throw new TypeError('tree must be an array.');
  
    if (!opts.weak || opts.strict) {
      tree.forEach((x) => {
        if (!(x instanceof Rule))
          throw new Error('tree may only contain Rules at the base');
      });
    }
  }
  
subject: Rename to render and add strict checking