static booleanOperator (condition) {
    if (Object.prototype.hasOwnProperty.call(condition, 'any')) {
      return 'any'
    } else if (Object.prototype.hasOwnProperty.call(condition, 'all')) {
      return 'all'
    } else if (Object.prototype.hasOwnProperty.call(condition, 'not')) {
      return 'not'
    }
  }
Returns the boolean operator for the condition
If the condition is not a boolean condition, the result will be 'undefined'
@return {string 'all', 'any', or 'not'}