Table schema:
CREATE TABLE table_name_18 (masculine_u_stems VARCHAR, neuter_a_stems VARCHAR, masculine_a_stems VARCHAR)

Question: What is the u form of the word with a neuter form of skip and a masculine a-ending of fisker?
SQL: SELECT masculine_u_stems FROM table_name_18 WHERE neuter_a_stems = "skip" AND masculine_a_stems = "fisker"