assets/js/vendor/backbone-relational.js
function( relation ) {
			return relation.instance instanceof this.relatedModel && this.reverseRelation.key === relation.key &&
				this.key === relation.reverseRelation.key;
		}
function( relation ) {
			return relation.instance instanceof this.relatedModel && this.reverseRelation.key === relation.key &&
				this.key === relation.reverseRelation.key;
		}
Determine if a relation (on a different RelationalModel) is the reverse
relation of the current one.
@param {Backbone.Relation} relation
@return {Boolean}