RangeType internal use

Hi,

I wonder why I cannot find a way to get a range type for a given (sub-)
type. I would like to build a RangeType from Datum's of lower and upper
bounds. Much like how construct_array() builds an ArrayType from a Datum
array of elements given elements' type info.

Is there some way I do not seem to know? If not, would it be worthwhile
to make something like construct_range() that returns a RangeType given
Datum's of lower and upper bounds and subtype info?

Thanks,
Amit