human
Considering the provided database schema and associated query, produce SQL code to retrieve the answer to the query. 
###Database Schema: Outcome, Date, Championship, Surface, Opponent, Score
###Question: who is the opponent when the surface is clay, the outcome is winner and the championship is estoril on 15 april 1996?
gpt
SELECT Opponent FROM table WHERE Surface = clay AND Outcome = winner AND Championship = estoril AND Date = 15 april 1996