shallow() casts its string argument to a seq and then tries to access
its fields. Guess what happens when that string is nil, which seems to
be the representation of an empty string (both the default value and an
explicitly assigned "").
Segfault encountered when running "ntags -R ." on a large project. The relevant line:
https://bitbucket.org/nimcontrib/ntags/src/a1c62c38e539877c105fbe4e08d06b76232f8017/ntags.nim#lines-125
