#diagonal.line  an oblique line of squares of the same color on a checkerboard; "the bishop moves on the diagonals"
  supertype:  #line.formation  #line
  part of:  #checkerboard
  subtype:  #main_diagonal__principal_diagonal__principaldiagonal  the diagonal of a square matrix running from the upper left entry to the lower right entry
  subtype:  #secondary_diagonal__secondarydiagonal  the diagonal of a square matrix running from the lower left entry to the upper right entry

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