code: function makeThresholdsSieve(options, defaults) {
  
  	if (!options) options = {};
  
  	// Pre-apply defaults.
  	Object.keys(defaults).forEach(key => options[key] || (options[key] = defaults[key]));
  
  	return (key, overrides) => {
  		if (overrides && overrides[key] !== void 0) return overrides[key];
  		return options[key];
  	}
  }
docs: This is used for classifying `time` and `gauge` values.