#tread  structural member consisting of the horizontal part of a stair or step
  supertype:  structural_member  support that is a constituent part of any structure or building
  part of:  stair__step  support consisting of a place to rest the foot while ascending or descending a stairway; "he paused on the bottom step"
     part of:  stairway__staircase__stairs__steps__step  a way of access consisting of a set of steps
        part of:  edifice__building  a structure that has a roof and walls and stands more or less permanently in one place; "there was a three-story building on the corner"; "it was an imposing edifice"

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