Someone reviewing an article of mine claims that this sentence begins with a dangling participle. Is that true? Is the meaning of the sentence ambiguous?
Before defining the derivative, it is useful to first define nullability.
The sentence does not even begin with a participial phrase, but with a prepositional phrase.
Ask someone if they think that "Before dinner, it is useful to define nullability" is a dangling participle as well.
And even more to the point, tell them that a dangling participle is not a mistake. It is just a label for a figure of speech.
A dangling participle is a common and useful shorthand that exists (in English and a great many other languages) precisely because it is useful.