Thank you! Thank you! I am one of those people who try desperately to point out the logic and they just won't listen.

It's too bad, in this one sense anyway, that we don't use parentheses as people do in math and programming, namely to group elements that are a unit -- something like 12(a + b). The structure of the example sentence might be clearer that way:

Mary Smith is one of (the librarians who oppose the contract).

I suppose sentence diagramming would do the trick as well ...

The issue can arise even without relative clauses. J.R.R. Tolkien once got a letter addressed to "any professor of English language" asking which was correct, A number of walls is being built or A number of walls are being built. (Big money was said to be riding on this issue.)

In general, the verb agrees in number with the head of the subject phrase, which is number, but in this case number is rather semantically bleached, and the walls are what is really at issue. He replied, therefore, that you can say what you like.

Yayyyyy! This is my ongoing cause. When I was a wee little beginning copy editor, one of the slots simply could not grasp this concept. There's nothing like knowing that you are absolutely right. Eventually, I believe, I found some written explanation, and after that -- well, I don't think she really understood it, but she'd ask me which one it ought to be, "has" or "have."

you know what's worse (?) than presecriptivists who refuse to budge from their moldy old rules? The "I told you so" people who get all puffed up with superiority when "their" cause, which had been challenged in the past, is validated by a third party. I don't remember when language became a contest to be deemed correct more often than the other guys.

-- from a reformed grammar nazi

..."when language became a contest to be deemed correct more often than the other guys."

Probably started about the time that the first guy scratched some marks on a rock.

Thank you, John. This is one I could not figure out on my own.

A relative prefers (if I may anthropomorphize words) to be close to its antecedent. "Mary, who opposes the contract, is one of the librarians" or "Mary is one who opposes the contract, of the librarians" reads MUCH more smoothly than "Mary is one, of the librarians, who opposes the contract" (commas added for clarity's sake, though they needn't be there) should that be what you want to say.

mike - that whole parenthesis business? Isn't that the numerical equivalent of diagramming?

"Mary is one who opposes the contract, of the librarians"

Whoa, Ridger...I'm not sure what you've been reading, but this sentence doesn't read more smoothly than anything!

Thank you for explaining it in such a simple way.