decorate (operator) {
    const next = operator.evaluate.bind(operator)
    return new Operator(
        `${this.name}:${operator.name}`,
        (factValue, jsonValue) => {
          return this.cb(factValue, jsonValue, next)
        },
        this.factValueValidator
    )
  }
Takes the fact result and compares it to the condition 'value', using the callback
@param   {Operator} operator - fact result
@returns {Operator} - whether the values pass the operator test