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

    my $interactive = -t STDOUT && -t STDIN;

    # Close out previous pager (if any):
    if(  $interactive  ) {
        select STDOUT;
        close PAGER;
    }

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

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

    return $response;
}