How to mark fields as non-expandable in yasnippet

Question

How do I selectively mark snippet fields as being expandable or non-expandable?

For example, I have a snippet

# name: read in vector
# key: cinvec
# --
std::vector<${1:_type_}> ${2:_name_}(${3:_size_});
for(int i = 0; i < $3; ++i) std::cin >> $2[i];

But I want the $2 field to be non-expandable because it is just a name. If I accidentally write another key into $2 and press TAB, it will do nested expansion instead of just going to the next field.