/usr/lib/aspell/spell is in libaspell15 0.60.7~20110707-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 | #!/bin/sh
# aspell list mimicks the standard unix spell program, roughly.
cat "$@" | aspell list --mode=none | sort -u
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