#twin  either of two offspring born at the same time from the same pregnancy
  supertype:  #sibling__sib  a person's brother or sister
  subtype:  #fraternal_twin__fraternaltwin__dizygotic_twin__dizygotictwin  either of two twins who developed from two separate fertilized eggs
  subtype:  #identical_twin__identicaltwin__monozygotic_twin__monozygotictwin__monozygoustwin  either of two twins developed from the same fertilized ovum (having the same genetic material)
     subtype:  #Siamese_twin__conjoined_twin__conjoinedtwin  one of a pair of identical twins born with their bodies joined at some point

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