Traverses directory structure and backtracks when necessary.

Example:

  ?- file(A/B),atom_concat(t,_,B).

True when A is unified to a subdirectory and B is unified to a filename that begins with a letter t.

 file(?Pathterm) is nondet

 filetype(?Pathterm_with_extension) is nondet

 dir(?PathTerm) is nondet

 pathterm_atom(++PathTerm, -Res) is det