Not quite, there's also ordering and indirectness. They've proved that all letters in the English alphabet have at least one word where either appending or prep

Not quite, there's also ordering and indirectness. They've proved that all letters in the English alphabet have at least one word where either appending or prepending that letter does not modify the pronunciation, or have at least one word where changing the first or last letter to the one in question does not modify the pronunciation and that these are not disjoint sets. Someone's silly Friday project, I think.