for each collection {
  $have-apple   = 0;
  $have-orange  = 0;
  $have-pear    = 0;

  #examine about 30 items:
  for each item {
    if( $item eq "apple" )  { $have-apple  = 1; }
    if( $item eq "orange" ) { $have-orange = 1; }
    if( $item eq "pear" )   { $have-pear   = 1; }
  }

  # then, when all items examined
  unless( $have-apple )  { add apple; }
  unless( $have-orange ) { add orange; }
  unless( $have-pear )   { add pear; }
}