// see docstring in .h file
std::string llsd_matches(const LLSD& prototype, const LLSD& data, const std::string& pfx)
{
    // An undefined prototype means that any data is valid.
    // An undefined slot in an array or map prototype means that any data
    // may fill that slot.
    if (prototype.isUndefined())
        return "";
    // A prototype array must match a data array with at least as many
    // entries. Moreover, every prototype entry must match the
    // corresponding data entry.
    if (prototype.isArray())
    {
        if (! data.isArray())
        {
            return STRINGIZE(colon(pfx) << "Array" << op << sTypes.lookup(data.type()));
        }
        if (data.size() < prototype.size())
        {
            return STRINGIZE(colon(pfx) << "Array size " << prototype.size() << op
                             << "Array size " << data.size());
        }
        for (LLSD::Integer i = 0; i < prototype.size(); ++i)
        {
            std::string match(llsd_matches(prototype[i], data[i], STRINGIZE('[' << i << ']')));
            if (! match.empty())
            {
                return match;
            }
        }
        return "";
    }
    // A prototype map must match a data map. Every key in the prototype
    // must have a corresponding key in the data map; every value in the
    // prototype must match the corresponding key's value in the data.
    if (prototype.isMap())
    {
        if (! data.isMap())
        {
            return STRINGIZE(colon(pfx) << "Map" << op << sTypes.lookup(data.type()));
        }
        // If there are a number of keys missing from the data, it would be
        // frustrating to a coder to discover them one at a time, with a big
        // build each time. Enumerate all missing keys.
        std::ostringstream out;
        out << colon(pfx);
        const char* init = "Map missing keys: ";
        const char* sep = init;
        for (LLSD::map_const_iterator mi = prototype.beginMap(); mi != prototype.endMap(); ++mi)
        {
            if (! data.has(mi->first))
            {
                out << sep << mi->first;
                sep = ", ";
            }
        }
        // So... are we missing any keys?
        if (sep != init)
        {
            return out.str();
        }
        // Good, the data block contains all the keys required by the
        // prototype. Now match the prototype entries.
        for (LLSD::map_const_iterator mi2 = prototype.beginMap(); mi2 != prototype.endMap(); ++mi2)
        {
            std::string match(llsd_matches(mi2->second, data[mi2->first],
                                           STRINGIZE("['" << mi2->first << "']")));
            if (! match.empty())
            {
                return match;
            }
        }
        return "";
    }
    // A String prototype can match String, Boolean, Integer, Real, UUID,
    // Date and URI, because any of these can be converted to String.
    if (prototype.isString())
    {
        static LLSD::Type accept[] =
        {
            LLSD::TypeBoolean,
            LLSD::TypeInteger,
            LLSD::TypeReal,
            LLSD::TypeUUID,
            LLSD::TypeDate,
            LLSD::TypeURI
        };
        return match_types(prototype.type(),
                           TypeVector(boost::begin(accept), boost::end(accept)),
                           data.type(),
                           pfx);
    }
    // Boolean, Integer, Real match each other or String. TBD: ensure that
    // a String value is numeric.
    if (prototype.isBoolean() || prototype.isInteger() || prototype.isReal())
    {
        static LLSD::Type all[] =
        {
            LLSD::TypeBoolean,
            LLSD::TypeInteger,
            LLSD::TypeReal,
            LLSD::TypeString
        };
        // Funny business: shuffle the set of acceptable types to include all
        // but the prototype's type. Get the acceptable types in a set.
        std::set<LLSD::Type> rest(boost::begin(all), boost::end(all));
        // Remove the prototype's type because we pass that separately.
        rest.erase(prototype.type());
        return match_types(prototype.type(),
                           TypeVector(rest.begin(), rest.end()),
                           data.type(),
                           pfx);
    }
    // UUID, Date and URI match themselves or String.
    if (prototype.isUUID() || prototype.isDate() || prototype.isURI())
    {
        static LLSD::Type accept[] =
        {
            LLSD::TypeString
        };
        return match_types(prototype.type(),
                           TypeVector(boost::begin(accept), boost::end(accept)),
                           data.type(),
                           pfx);
    }
    // We don't yet know the conversion semantics associated with any new LLSD
    // data type that might be added, so until we've been extended to handle
    // them, assume it's strict: the new type matches only itself. (This is
    // true of Binary, which is why we don't handle that case separately.) Too
    // bad LLSD doesn't define isConvertible(Type to, Type from).
    return match_types(prototype.type(), TypeVector(), data.type(), pfx);
}