#keystone__headstone  the central building block at the top of an arch or vault
  supertype:  #building_block.building_material__buildingblock  a block of material used in construction work
  subtype:  #quoin  the keystone of an arch
  part of:  #arch  (architecture) a masonry construction (usually curved) for spanning an opening and supporting the weight above it
     part of:  #aqueduct  a conduit that resembles a bridge but carries water over a valley
     part of:  #arcade.structure__colonnade  a structure composed of a series of arches supported by columns
     part of:  #bridge__span  a structure that allows people or vehicles to cross an obstacle such as a river or canal or railway etc.
        part of:  #transportation_system__transportation__transit  facility consisting of the roads and equipment necessary for the movement of passengers or goods
           part of:  #infrastructure__base  the stock of basic facilities and capital equipment needed for the functioning of a country or area; "the industrial base of Japan"

No statement uses or specializes #keystone; click here to add one.