public <U> MultiExpression<U> mapResults(Function<T, U> fn) {
    return new MultiExpression<>(
      expressions.stream()
        .map(entry -> entry(fn.apply(entry.result), entry.expression))
        .filter(entry -> entry.result != null)
        .toList()
    );
  }
Returns a copy of this multi-expression, replacing the data value with {@code fn}.
planetiler-core/src/main/java/com/onthegomap/planetiler/expression/MultiExpression.java