code: function expandFilter( that, trigger ) {
  			that.debug.groupStart( 'expandFilter' ); // %REMOVE_LINE%
  
  			var upper = trigger.upper,
  				lower = trigger.lower;
  
  			if ( !upper || !lower 											// NOT: EDGE_MIDDLE trigger ALWAYS has two elements.
  				|| isFlowBreaker( lower ) || isFlowBreaker( upper )			// NOT: one of the elements is floated or positioned
  				|| lower.equals( upper ) || upper.equals( lower ) 			// NOT: two trigger elements, one equals another.
  				|| lower.contains( upper ) || upper.contains( lower ) ) { 	// NOT: two trigger elements, one contains another.
  				that.debug.logEnd( 'REJECTED. No upper or no lower or they contain each other.' ); // %REMOVE_LINE%
  
  				return false;
  			}
  
  			// YES: two trigger elements, pure siblings.
  			else if ( isTrigger( that, upper ) && isTrigger( that, lower ) && areSiblings( that, upper, lower ) ) {
  				that.debug.logElementsEnd( [ upper, lower ], // %REMOVE_LINE%
  					[ 'upper', 'lower' ], 'APPROVED EDGE_MIDDLE' ); // %REMOVE_LINE%
  
  				return true;
  			}
  
  			that.debug.logElementsEnd( [ upper, lower ], // %REMOVE_LINE%
  				[ 'upper', 'lower' ], 'Rejected unknown pair' ); // %REMOVE_LINE%
  
  			return false;
  		}
docs: A method for trigger filtering. Accepts or rejects trigger pairs by their location in DOM etc.