constant len : func(0, [Tree; int])

bind 0 x : {v : int  | true} 
bind 1 x : {v : Tree | true} 
bind 2 y : {v : int  | v = len x } 

constraint:
  env [ ]
  lhs {v : Tree | 666 < len v }
  rhs {v : Tree | 66  < len v }
  id 1 tag []