sub EXISTS   {
  my ($a,$key) = (shift, shift);

  # STEP 1
  return 1 if exists $a->[0]{$key}; # Have data

  # STEP 2
  my $cache = $a->[1]{$key};
  return $cache if defined $cache; # Existence cache

  # STEP 3
  my @res = $a->[3]($key,$a->[4]);
  $_[0][1]{$key} = 0, return unless @res; # Cache non-existence

  # STEP 4
  # Now we know it exists
  return ($_[0][1]{$key} = 1) if $a->[5]; # Only existence reported

  # STEP 5
  # Now know the value
  $_[0][0]{$key} = $res[0];     # Store data
  return 1
}