Write entails/necessarily involves Spell. Spell is a synonym of Interval. Interval is a synonym of Season. Season is related to Summer. 
Unit is related to Letter. Letter is related to Brief. Brief is related to Explain. Explain is related to Discuss. 
Write --Entails--> Spell <--Synonym--> Interval <--Synonym--> Season <--RelatedTo--> Summer 
Unit <--RelatedTo--> Letter <--RelatedTo--> Brief <--RelatedTo--> Explain <--RelatedTo--> Discuss 