data LL 1 = [
  | emp { }
  | con { lHead : @(0), lTail : LL @(0) }
]

bind 1 undef : {v: func(1, [@(0)]) | []}

constraint:
  env [1]
  lhs {v : LL int | [v = undef]}
  rhs {v : LL int | [0 < 7]}
  id 1 tag []