#element.straight_line  a straight line that generates a cylinder or cone
  supertype:  #straight_line  a line traced by a point traveling in a constant direction; a line of zero curvature; "the shortest distance between two points is a straight line"
  subtype:  #element_of_a_cone  a straight line joining the apex and a point on the base
  subtype:  #element_of_a_cylinder  a straight line running the length of the cylinder

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