#open_chain  a chain of atoms in a molecule whose ends are not joined to form a ring
  exclusion:  closed_chain
  supertype:  chemical_chain__chain  a series of linked atoms (generally in an organic molecule)
  subtype:  branched_chain  an open chain of atoms with one or more side chains attached to it
  subtype:  straight_chain  an open chain of atoms with no side chains

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