#seal.holdfast  fastener that provides a tight and perfect closure
  supertype:  holdfast__fastener__fastening  restraint that attaches to something or holds something in place
  subtype:  break_seal__breakseal  a seal that must be broken when first used and cannot easily be resealed; "it was stored in a tube with a break seal"
  subtype:  gasket  seal consisting of a ring for packing pistons or sealing a pipe joint
     subtype:  head_gasket  a gasket to seal a cylinder head
     subtype:  O_ring  a gasket consisting of a flat ring of rubber or plastic; used to seal a joint against high pressure
  subtype:  piston_ring  seal consisting of a split metal ring that seals the gap between a piston and the cylinder wall
  subtype:  washer  seal consisting of a flat disk placed to prevent leakage
     subtype:  lockring__lock_washer  washer that prevents a nut from loosening

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