code: function getPopulation( props ) {
    // extract all the population values as numbers and find the first non-negative value
    // returns undefined if there are no such values
    return population_hierarchy.
            map((field) => { return _.toNumber(props[field]); }).
            find((val) => { return val >= 0; } );
  }
docs: this function favors mz:population when available, falling back to other properties. see: https://github.com/whosonfirst-data/whosonfirst-data/issues/240#issuecomment-294907374