sub new {
    # SNIP

    # BEGIN vicious error checking
    #   all input must be legitimate
    $x{'perm'}=[ map { $_-1 if defined and /^\d+$/ and $_ > 0} @_ ];
    return undef unless $#{$x{'perm'}} == $#_;
    #   it must really be a permutation of 0..n-1
    return undef unless join(' ',sort { $a <=> $b } @{$x{'perm'}}) eq join(' ',0..$#{$x{'perm'}});
    # END error checking

    # SNIP

    $x{'n'} = scalar @{$x{'perm'}}; # for clarity in other places

    bless \%x, $class;
}