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The lexicon of #TheLanguageOfEverythingElse has only word-stems, but all stems have a variant usable as an affix.
If "rational" is the product of "ration" (noun) + "-al" (postfix for adjectifying a noun), then you can also use a variant of "al" as a noun and "-ration-" as an affix.
For example, if "ration-al" is syntactically legal, then "al-ration" must be too—perhaps referring to something like "the act of adjectifying something in a manner that accentuates the speaker's reasoning ability".
In general, any word can:
1) be inflected into any word-category (eg "Hanson" ↦ "hansonian", "hansoning", "hansonium", "potato-hanson")
2) be a word-category that other words inflect into (eg "potato" ↦ "hanson-potato", "adjective" ↦ "hanson-adjective")
Furthermore, you can apply affixes unto any multiword segment of a sentence by wrapping it within "|"s. If you want to, you could say eg "I am an anti|washing under the couch|ist"—meaning "I don't want to wash under the couch, and this fact is part of my identity".