From what I understand, すら is commonly used with the negative conjugation (~ない, ~なかった) of verbs.
「ひらがなすら書けない。」 I expected (him) to be able to write hiragana. But even hiragana, (he) can't write.
「涙すら出なかった。」 I expected tears to fall. But even tears, didn't fall. (I expect him to cry but he didn't even wept a tear.)
Basically I was wondering is it grammatical to use すら with the positive conjugation of a verb?
For example, can 「涙すら出た。」 be considered grammatical, assuming the context is that the speaker expected "no tears to fall", but indeed "tears did fall"?