docs: function() {
      var captures;
      if (captures = /^while +([^\n]+)/.exec(this.input)) {
        this.consume(captures[0].length);
        this.assertExpression(captures[1])
        this.tokens.push(this.tok('while', captures[1]));
        return true;
      }
      if (this.scan(/^while\b/)) {
        this.error('NO_WHILE_EXPRESSION', 'missing expression for while');
      }
    }