Should liblouis display tables be discoverable?

[bertfrees]

See 3e62d13

At the moment they are not (the official Liblouis tables).

The problem is that when a display table is not explicitly requested no display table should be returned, which requires matching for absence of type:display, which is not possible with the current Liblouis query syntax.

Before commit 3e62d13 the problem was solved by matching type:translation. The downside of this was that the feature had to be added to every single table.

[bertfrees]

Update: see liblouis/liblouis#541