Table schema:
CREATE TABLE table_name_18 (masculine_an_stems VARCHAR, feminine_ō_stems VARCHAR, masculine_u_stems VARCHAR)

Question: What is the an-stem for the word which has an ö-stems of siangar and an u-stem ending of syni?
SQL: SELECT masculine_an_stems FROM table_name_18 WHERE feminine_ō_stems = "siangar" AND masculine_u_stems = "syni"