How can a proof question be asked online? Let us consider a quiz question whose answer is a proof. For instance, a question in mathematics, such as prove that for every natural number $n$ the quantity $n^3-n$ is even.

How could such a question be implemented in a computer interface so that the computer can check the proof for correctness?

A simple, yet unreasonable way would be to have a multiple choice format for the question, where every choice was one way of proving the relationship, but I guess the shortcomings of this are obvious...
I have experimented with a system that provides students with a collection of phrases/formulae from which they can drag and drop a selection to construct a proof.  I think that this is a promising approach but there are still a large number of different ways in which students can get the answer wrong, and an even larger number of ways that they can construct an answer that cannot be parsed as something meaningful.  If you simply reject such answers without comprehensible feedback then you will just make the students hate you.  So you have to write a large amount of code that tries to analyse all possible answers and explain what (if anything) is wrong with them.  The logic is quite complex and I am not sure how well the students would understand the explanations.  I hope to return to these experiments at some point but at the moment I am not teaching anything for which they would be useful.