Return a dictionary with unique prefixes for modules in `context`.

    Keys are 'module' statements and values are prefixes,
    disambiguated where necessary.

def unique_prefixes(context):
    """Return a dictionary with unique prefixes for modules in `context`.

    Keys are 'module' statements and values are prefixes,
    disambiguated where necessary.
    """
    res = {}
    for m in context.modules.values():
        if m.keyword == "submodule": continue
        prf = new = m.i_prefix
        suff = 0
        while new in res.values():
            suff += 1
            new = "%s%x" % (prf, suff)
        res[m] = new
    return res