Markdown to markdown pre-filtering for equation numbers loses YAML meta block

["R (Chandra) Chandrasekhar" <chyavana-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

I need to assign equation numbers in a markdown document and am running 
the pandoc-eqnos filter specifically to get a source document with that. 
Ergo, I need the output format to be markdown as well, preferably the 
default flavour put out by pandoc.

I tried

pandoc --to markdown --filter pandoc-eqnos --output out.md in.md

but I find that the YAML metadada block at the very top of in.md 
disappears in the out.md. What should I do to preserve it?

Thanks.

Chandra