create or alter procedure SOME$PROC$Ok as
declare variable I integer;
begin
  with recursive aTree as (select 1 as aLevel
                           from rdb$database
                           union all
                           select T.aLevel + 1 as aLevel
                           from aTree t)
  select first 1 * from aTree into :I;  
end

create or alter procedure SOME$PROC$Error as
declare variable I integer;
begin  
  I = (with recursive aTree as (select 1 as aLevel
                           from rdb$database
                           union all
                           select T.aLevel + 1 as aLevel
                           from aTree t)
       select first 1 * from aTree);
end

This operation is not defined for system tables.
unsuccessful metadata update.
GRANT failed.
Table ATREE does not exist.