You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Martin_Luther

Context:
Luther wrote "Ach Gott, vom Himmel sieh darein" ("Oh God, look down from heaven"). "Nun komm, der Heiden Heiland" (Now come, Savior of the gentiles), based on Veni redemptor gentium, became the main hymn (Hauptlied) for Advent. He transformed A solus ortus cardine to "Christum wir sollen loben schon" ("We should now praise Christ") and Veni Creator Spiritus to "Komm, Gott Schöpfer, Heiliger Geist" ("Come, Holy Spirit, Lord God"). He wrote two hymns on the Ten Commandments, "Dies sind die heilgen Zehn Gebot" and "Mensch, willst du leben seliglich". His "Gelobet seist du, Jesu Christ" ("Praise be to You, Jesus Christ") became the main hymn for Christmas. He wrote for Pentecost "Nun bitten wir den Heiligen Geist", and adopted for Easter "Christ ist erstanden" (Christ is risen), based on Victimae paschali laudes. "Mit Fried und Freud ich fahr dahin", a paraphrase of Nunc dimittis, was intended for Purification, but became also a funeral hymn. He paraphrased the Te Deum as "Herr Gott, dich loben wir" with a simplified form of the melody. It became known as the German Te Deum.

Question:
What is a Hauptlied?

main hymn