YES

Problem:
 flatten(nil()) -> nil()
 flatten(unit(x)) -> flatten(x)
 flatten(++(x,y)) -> ++(flatten(x),flatten(y))
 flatten(++(unit(x),y)) -> ++(flatten(x),flatten(y))
 flatten(flatten(x)) -> flatten(x)
 rev(nil()) -> nil()
 rev(unit(x)) -> unit(x)
 rev(++(x,y)) -> ++(rev(y),rev(x))
 rev(rev(x)) -> x
 ++(x,nil()) -> x
 ++(nil(),y) -> y
 ++(++(x,y),z) -> ++(x,++(y,z))

Proof:
 Matrix Interpretation Processor: dim=1
  
  interpretation:
   [rev](x0) = 2x0,
   
   [++](x0, x1) = x0 + x1,
   
   [unit](x0) = 2x0,
   
   [flatten](x0) = 4x0,
   
   [nil] = 5
  orientation:
   flatten(nil()) = 20 >= 5 = nil()
   
   flatten(unit(x)) = 8x >= 4x = flatten(x)
   
   flatten(++(x,y)) = 4x + 4y >= 4x + 4y = ++(flatten(x),flatten(y))
   
   flatten(++(unit(x),y)) = 8x + 4y >= 4x + 4y = ++(flatten(x),flatten(y))
   
   flatten(flatten(x)) = 16x >= 4x = flatten(x)
   
   rev(nil()) = 10 >= 5 = nil()
   
   rev(unit(x)) = 4x >= 2x = unit(x)
   
   rev(++(x,y)) = 2x + 2y >= 2x + 2y = ++(rev(y),rev(x))
   
   rev(rev(x)) = 4x >= x = x
   
   ++(x,nil()) = x + 5 >= x = x
   
   ++(nil(),y) = y + 5 >= y = y
   
   ++(++(x,y),z) = x + y + z >= x + y + z = ++(x,++(y,z))
  problem:
   flatten(unit(x)) -> flatten(x)
   flatten(++(x,y)) -> ++(flatten(x),flatten(y))
   flatten(++(unit(x),y)) -> ++(flatten(x),flatten(y))
   flatten(flatten(x)) -> flatten(x)
   rev(unit(x)) -> unit(x)
   rev(++(x,y)) -> ++(rev(y),rev(x))
   rev(rev(x)) -> x
   ++(++(x,y),z) -> ++(x,++(y,z))
  Matrix Interpretation Processor: dim=1
   
   interpretation:
    [rev](x0) = 4x0,
    
    [++](x0, x1) = x0 + x1,
    
    [unit](x0) = x0 + 4,
    
    [flatten](x0) = 4x0
   orientation:
    flatten(unit(x)) = 4x + 16 >= 4x = flatten(x)
    
    flatten(++(x,y)) = 4x + 4y >= 4x + 4y = ++(flatten(x),flatten(y))
    
    flatten(++(unit(x),y)) = 4x + 4y + 16 >= 4x + 4y = ++(flatten(x),flatten(y))
    
    flatten(flatten(x)) = 16x >= 4x = flatten(x)
    
    rev(unit(x)) = 4x + 16 >= x + 4 = unit(x)
    
    rev(++(x,y)) = 4x + 4y >= 4x + 4y = ++(rev(y),rev(x))
    
    rev(rev(x)) = 16x >= x = x
    
    ++(++(x,y),z) = x + y + z >= x + y + z = ++(x,++(y,z))
   problem:
    flatten(++(x,y)) -> ++(flatten(x),flatten(y))
    flatten(flatten(x)) -> flatten(x)
    rev(++(x,y)) -> ++(rev(y),rev(x))
    rev(rev(x)) -> x
    ++(++(x,y),z) -> ++(x,++(y,z))
   Matrix Interpretation Processor: dim=1
    
    interpretation:
     [rev](x0) = 2x0 + 2,
     
     [++](x0, x1) = x0 + x1 + 2,
     
     [flatten](x0) = 2x0
    orientation:
     flatten(++(x,y)) = 2x + 2y + 4 >= 2x + 2y + 2 = ++(flatten(x),flatten(y))
     
     flatten(flatten(x)) = 4x >= 2x = flatten(x)
     
     rev(++(x,y)) = 2x + 2y + 6 >= 2x + 2y + 6 = ++(rev(y),rev(x))
     
     rev(rev(x)) = 4x + 6 >= x = x
     
     ++(++(x,y),z) = x + y + z + 4 >= x + y + z + 4 = ++(x,++(y,z))
    problem:
     flatten(flatten(x)) -> flatten(x)
     rev(++(x,y)) -> ++(rev(y),rev(x))
     ++(++(x,y),z) -> ++(x,++(y,z))
    Matrix Interpretation Processor: dim=1
     
     interpretation:
      [rev](x0) = 3x0,
      
      [++](x0, x1) = x0 + x1 + 3,
      
      [flatten](x0) = 4x0 + 1
     orientation:
      flatten(flatten(x)) = 16x + 5 >= 4x + 1 = flatten(x)
      
      rev(++(x,y)) = 3x + 3y + 9 >= 3x + 3y + 3 = ++(rev(y),rev(x))
      
      ++(++(x,y),z) = x + y + z + 6 >= x + y + z + 6 = ++(x,++(y,z))
     problem:
      ++(++(x,y),z) -> ++(x,++(y,z))
     Matrix Interpretation Processor: dim=1
      
      interpretation:
       [++](x0, x1) = 2x0 + x1 + 1
      orientation:
       ++(++(x,y),z) = 4x + 2y + z + 3 >= 2x + 2y + z + 2 = ++(x,++(y,z))
      problem:
       
      Qed