vendors/execa.js/add
      function add(source) {
        if (Array.isArray(source)) {
          source.forEach(add);
          return this;
        }
        sources.push(source);
        source.once("end", remove.bind(null, source));
        source.once("error", output.emit.bind(output, "error"));
        source.pipe(output, { end: false });
        return this;
      }