sub install_method {
        # special class method called directly by apps and/or drivers
        # to install new methods into the DBI dispatcher
        # DBD::Foo::db->install_method("foo_mumble", { usage => [...], options => '...' });
        my ($class, $method, $attr) = @_;
        Carp::croak("Class '$class' must begin with DBD:: and end with ::db or ::st")
            unless $class =~ /^DBD::(\w+)::(dr|db|st)$/;
        my ($driver, $subtype) = ($1, $2);
        Carp::croak("invalid method name '$method'")
            unless $method =~ m/^([a-z][a-z0-9]*_)\w+$/;
        my $prefix = $1;
        my $reg_info = $dbd_prefix_registry->{$prefix};
        Carp::carp("method name prefix '$prefix' is not associated with a registered driver") unless $reg_info;

        my $full_method = "DBI::${subtype}::$method";
        $DBI::installed_methods{$full_method} = $attr;

        my (undef, $filename, $line) = caller;
        # XXX reformat $attr as needed for _install_method
        my %attr = %{$attr||{}}; # copy so we can edit
        DBI->_install_method("DBI::${subtype}::$method", "$filename at line $line", \%attr);
    }