import re


def find_cites(f):
    """Return keys to cited papers.

    """
    with open(f) as fp:
        lst = re.findall(r"{{(.+?)}}", fp.read())
    refs = []
    for l in lst:
        if "site.data.refs" in l:
            refs.append(l.split(".")[3])
    return sorted(set(refs))