YES(?,O(n^2))

Problem:
 ++(nil(),y) -> y
 ++(x,nil()) -> x
 ++(.(x,y),z) -> .(x,++(y,z))
 ++(++(x,y),z) -> ++(x,++(y,z))

Proof:
 Matrix Interpretation Processor:
  dimension: 2
  interpretation:
                 [1 2]          [10]
   [.](x0, x1) = [0 1]x0 + x1 + [4 ],
   
                  [1 2]          [8]
   [++](x0, x1) = [0 1]x0 + x1 + [2],
   
           [4]
   [nil] = [7]
  orientation:
                     [26]         
   ++(nil(),y) = y + [9 ] >= y = y
   
                 [1 2]    [12]         
   ++(x,nil()) = [0 1]x + [9 ] >= x = x
   
                  [1 4]    [1 2]        [26]    [1 2]    [1 2]        [18]               
   ++(.(x,y),z) = [0 1]x + [0 1]y + z + [6 ] >= [0 1]x + [0 1]y + z + [6 ] = .(x,++(y,z))
   
                   [1 4]    [1 2]        [20]    [1 2]    [1 2]        [16]                
   ++(++(x,y),z) = [0 1]x + [0 1]y + z + [4 ] >= [0 1]x + [0 1]y + z + [4 ] = ++(x,++(y,z))
  problem:
   
  Qed