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In general , if " R " is a ring and " S " is a simple module over " R " , then , by Schur 's lemma , the endomorphism ring of " S " is a division ring ; every division ring arises in this fashion from some simple module .
 Schur
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Schur