rw [removePosAssignment, addPosAssignment]
assignment : Assignment
h : ¬assignment.hasPosAssignment = true
⊢ assignment.addPosAssignment.removePosAssignment = assignment
assignment : Assignment
h : ¬assignment.hasPosAssignment = true
⊢ (match
      match assignment with
      | pos => pos
      | neg => both
      | both => both
      | unassigned => pos with
    | pos => unassigned
    | neg => neg
    | both => neg
    | unassigned => unassigned) =
    assignment