docs: function Constraint ( data, options ) {
      if ( ! ( this instanceof Constraint ) ) {
        return new Constraint( data, options );
      }
  
      this.__class__ = 'Constraint';
      this.options = options || {};
      this.nodes = {};
  
      if ( data ) {
        try {
          this._bootstrap( data );
        } catch ( err ) {
          throw new Error( 'Should give a valid mapping object to Constraint', err, data );
        }
      }
    }