bind 0 x  : {VV : Tree | [] }
bind 1 x  : {VV : List | [] }
bind 2 x  : {VV : real | [] }

constraint:
  env [0]
  lhs {VV : Tree | [ VV = x ] }
  rhs {VV : Tree | [ VV = x ] }
  id 1 tag [1]