sub getline
  { return unless @_;                              # caller don't want checks
    my %valid_word= map { ( $_, 1) } @_;           # create a hash valid_word => 1
    my $line;
    chomp( $line = <>);
    my @word= split /\s/, $line;
    foreach my $word (@word)
      { return undef unless $valid_word{$word}; }  # return failure as soon as a a bad word is found
    return $line;
  }