code: function compareBy(prop, priorValue, a, b) {
      const incA = includes(priorValue, a[prop]);
      const incB = includes(priorValue, b[prop]);
      if (incA && incB) return 0;
      if (!incA && !incB) return null; // neither of a or b
      if (incA) return -1;
      return 1; // incB
  }
docs: Compares the given two symbols by the specified property.
  @private
  
  @param {String} prop - Property/key to be used for comparison.
  @param {String|Array} priorValue - Priority value. Which ever symbol has it,
  comes first. This can also be an array of values.
  @param {Object} a - First symbol to be compared.
  @param {Object} b - Second symbol to be compared.
  
  @returns {Number} - `0` if both have the same value for the given
  property/key. `-1` if only `a` has the priority value. `1` if only
  `b` has the priority value. `null` if neither of the symbols have the
  priority value.