CREATE TABLE table_name_18 (masculine_u_stems VARCHAR(255), neuter_a_stems VARCHAR(255), masculine_a_stems VARCHAR(255))
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"