#variable.symbol  a symbol (like x or y) that is used in mathematical or logical expressions to represent a variable quantity
  supertype:  symbol  an arbitrary sign (written or printed) that has acquired a conventional significance
  subtype:  placeholder  a symbol in a logical or mathematical expression that can be replaced by the name of any member of specified set
  subtype:  unknown__unknown_quantity__unknownquantity  a variable whose values are solutions of an equation

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