Elpi Accumulate <db> Db <db> does not work #889

README.md states that databases can be accumulated into other databases:

Elpi Accumulate [<dbname>|<qname>] [<code>|File [Signature] <fname>|Db [Header] <dbname>]

AFAICT this was never actually supported and <dbname>|<qname> is always interpreted as an elpi program name in this command:

From elpi Require Import elpi.
Elpi Db test1 lp:{{ }}.
Elpi Db test2 lp:{{ }}.
Elpi Accumulate test1 Db test2. (* No Elpi Program named test1 *)

I've tried fixing this myself but the fact that all of this goes through mlg files breaks my OCaml backtraces and I cannot figure out the chain from the grammar rules to Interp.accumulate which seems to be responsible for the error message. I tried fixing that function directly but adding support for accumulating a db requires more arguments that I don't know how to get.