sub new
     {
     #Arg0 is the type because the constructor will look like
     #  my($instance) = Tree->new(arg1,arg2,whatever)
     #so arg0 will be Tree.
     my($type) = $_[0];

     #Make subroutine-local var $self, and make it a reference.
     #Specifically, make it a reference to a (right now) empty hash.
     #Later on, that hash will contain object properties.
     my($self) = {};

     #For now, we'll have one instance variable (property, whatever)
     #It will be in the hash referenced by $self, and will have
     #the index 'root'. This will be the first arg (inside the parentheses)
     #of the call to the constructor in the main program.
     $self->{'root'} = $_[1];    #remember $_[0] was the Tree before the ->

     #There's nothing reserved about the word $self. It could have been
     #called $oodolaboodola. To link the object with both the hash pointed
     #to by $self and the type (Tree), we use the 2 argument version
     #of the keyword bless:
     bless($self, $type);

     #Now finally, return the hash as a reference to be used as an "object"
     return($self);
     }