use strict;
use warnings;
use Group::Permutation;

my $x = new Group::Permutation qw(1 3 2 4);
print "x = $x\n";
print 'x of 2 = ',$x->of(2),"\n";
print 'ref of x of 2: ', ref $x->of(2),"\n";
print "x of 2 is three!\n" if ($x->of(2) == 3);

##</code><code>##

x = (1, 3, 2, 4)
x of 2 = 3
ref of x of 2: 

##</code><code>##

sub of {
    my $x = shift;

    my @y = map { $_-1 if defined && /^\d+$/ && $_>0 } @_;
    # protect the caller from placement errors
    return undef unless ($#y == $#_);

    return map { ($_<$$x{'n'}) ? $$x{'perm'}[$_] + 1 : $_+1 } @y;
}

##</code><code>##

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;
}