my $m = MyClass->new;
  my $a, $x;

  $a = $m->a;       # *undef*
  $x = $m->a_isset; # false
  $a = $m->a(1);    # 1
  $m->a(3);
  $x = $m->a_isset; # true
  $a = $m->a;       # 3
  $a = $m->a(5);     # 5;
  $m->a_reset;
  $x = $m->a_isset; # false