sub file_name {
	my ($client_number, $MM, $DD, $S) = @_;
	my @varnames = qw($client_number $MM $DD $S);
	for (my $i=0; $i <= scalar @_; $i++) {
		die "$varnames[$i] not defined\n" unless defined($_[$i]);
	}
	"I$client_number$MM$DD$S";
}