here explicitly variables are introduced

=> these range over a sequence of values

# return all cities in which conferences happen
for $city in // city
   for $conf in // conferences
   where
      happens ( event: $conf, location: $city, ... )
   return
      ( $city )

is down-translated into:

  [[ [ happens ( ... $conf ... $city ... ) ] ]] ( $city ) | $conf => [[ // conferences ]]
                                                          | $city => [[ // city ]]
# eliminating variable $conf
= [[ // conferences ]] .
  [[ [ happens ( ... $0 ... $city ... ) ] ]] ( $city )    | $city => [[ // city ]]
# eliminating variable $city
= [[ // city ]] .
  ( $0, [[ // conferences ]] ) .
  [[ [ happens ( ... $1 ... $0 ... ) ] ]] ( $0 )
# look ma: no variables!