#sign.mathematical_notation  a character indicating a relation between quantities; "don't forget the minus sign"
  supertype:  #mathematical_notation  a notation used by mathematicians
  subtype:  #equals_sign  a sign indicating that the quantities on either side are equal
  subtype:  #plus_sign  a sign indicating the operation of addition
  subtype:  #minus_sign__minussign  a sign indicating the operation of subtraction

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