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If the spell program has a different name or is not in your path, you can specify an alternative with set_spell_cmd. Any command that takes text from standard input and prints a list of misspelled words, one per line, to standard output will do. For example, you can use aspell -l."
You may want to take note that (apparently) for some versions of aspell it’s 'aspell list' and not 'aspell -l'
Hope this helps.