One question about index implementation:

Some database systems implement the b-tree indexes making it bidirectional,
so a bidirectional index uses less space than two unidirectional indexes.

Why Interbase doesn't implements bidirectional indexes?
- Because the sql standars says
CREATE [ASCENDING|DESCENDING] INDEX ON ...?
- Because it's dificult to update because the multigenerational
architechture?

Just curious.

Thanks