A path to a directory as defined by the operating system.
A number in the range 0 to 2:
0 Find all files and directories.
1 Find all files.
2 Find all directories.
If non-nil, show the full pathname of the file by prepending the path to all filenames.
Gamma> directory("/usr",0,nil);
("local" "lib" "bin" "readme")
Gamma> sort(directory("/usr",2,t),strcmp);
("/usr/bin" "/usr/lib" "/usr/local")