@thedansimonson @Miriamm But "be" is the primary English copular. It's embedded in every infinitive, so daring "it's a cipular verb, not an infinitive" doesn't make sense to me. And the infinitive structure – "is purple" – is right there on the sign. The modifier, "clearly," – again, if you're a semi-purist like me; that's another discussion – therefore doesn't go between "is" and "onion."
Also, my original tweet was a joke.
@msbellows @thedansimonson @Miriamm Infinitive... You keep using that word. I don't think it means what you think it means.