How to type text over rightarrow in Formula Editor

I want to type the the term “a.s.” over rightarrow in Writer’s Formula Editor giving it the following look:
image description

I tried “a.s. over rightarrow”, but this does not work. How is it possible to achieve this formatting?

Try { a.s.} csub {widevec {~~~~~} } - you can change the number of ~s to grow the size of the arrow.

Thank you, this trick is really useful to lengthen the arrow underneath.

You can use rightarrow csup "a.s." or toward csup "a.s.". But up to now the arrow is not stretchy, see bug https://bugs.documentfoundation.org/show_bug.cgi?id=34563.