#barrier.mechanism  anything serving to maintain separation by obstructing vision or access
  supertype:  #mechanism.natural_object  a natural object resembling a machine in structure and function; "the mechanism of the ear"
  subtype:  #bar.ridge  a submerged (or partly submerged) ridge in a river or along a shore
     subtype:  #sandbar__sand_bar  a bar of sand
  subtype:  #blood-brain_barrier  a mechanism that creates a barrier between brain tissues and circulating blood; serves to protect the central nervous system; "the brain was protected from the large molecules of the virus by the blood-brain barrier"
  subtype:  #curtain  any barrier to communication or vision; "a curtain of secrecy"; "a curtain of trees"

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