$ cat > /tmp/t.sql 
create table t0 (a integer, af computed by (a*3) default 10);

$ isql t.fdb -i /tmp/t.sql
Statement failed, SQLSTATE = 42000
Dynamic SQL Error
-SQL error code = -104
-Token unknown - line 1, column 50
-default
After line 0 in file /tmp/t.sql

At line 1 in file /tmp/t.sql