sub prompt {
    my( $prompt, $validator, $reprompt ) = @_;

    # Close out previous pager (if any):
    select STDOUT;
    close PAGER;

    # Prompt the user for a response:
    my $response;
    while(  1  ) {
        print STDERR $prompt;
        $response = <STDIN>;
        die "End of input.\n"
            if  ! defined $response;
        chomp $response;
        last
            if  $validator->( $response );
        $prompt = $reprompt;
    }

    # Run output (until next prompt) through pager:
    my $pager = $ENV{PAGER} || 'less';
    open PAGER, '|-', $pager
        or  die "Can't run $pager: $!\n";
    select PAGER;

    return $response;
}