Interval has a make_unbounded call. If this make routine is called
then there is no test to check if the interval remains in this state.
If it does, then the as_string routine provides a single bound as does
the out routine. This is compounded in C_REAL and may be in other classes
that use the interval class.
It may be best to leave it like this if there is no suitable syntax for an
unbounded interval - but the make_unbounded should probably not be present.