It's legal for tag names to begin with a number, right? It works fine
in all other respects but in AQL it gives a syntax error.
E.g. at WormBase, http://www.wormbase.org/db/searches/aql_query,
"select U from U in class UTR where exists_tag U->5_UTR" gives
"AQL error 700 around: '_tag U->5_UTR"