code: function unEquip (attributes, slotName) {
          if (!(slotName in attributes.equipped)) {
              throw new SlotNameError(slotName);
          }
  
          if (attributes.equipped[slotName]) {
              attributes.inventory.push(attributes.equipped[slotName]);
          }
  
          attributes.equipped[slotName] = null;
  
          return attributes;
      }
docs: Unequips item from given slotName
  @param {Attributes} attributes which object should be operated. In Mongoose it this, in Backbone it's this.attributes
  @param {String} slotName