Index is related to Finger. Finger is related to Long. Long is related to Whip. 
Extensive is a synonym of Extended. Extended is related to Stretch. Stretch is related to Gentle. 
Index <--RelatedTo--> Finger <--RelatedTo--> Long <--RelatedTo--> Whip 
Extensive <--Synonym--> Extended <--RelatedTo--> Stretch <--RelatedTo--> Gentle 