We consider the following Problem:

  Strict Trs:
    {  sum(cons(s(n), x), cons(m, y)) -> sum(cons(n, x), cons(s(m), y))
     , sum(cons(0(), x), y) -> sum(x, y)
     , sum(nil(), y) -> y
     , weight(cons(n, cons(m, x))) ->
       weight(sum(cons(n, cons(m, x)), cons(0(), x)))
     , weight(cons(n, nil())) -> n}
  StartTerms: basic terms
  Strategy: innermost

Certificate: YES(?,O(n^2))

Proof:
  We consider the following Problem:
  
    Strict Trs:
      {  sum(cons(s(n), x), cons(m, y)) -> sum(cons(n, x), cons(s(m), y))
       , sum(cons(0(), x), y) -> sum(x, y)
       , sum(nil(), y) -> y
       , weight(cons(n, cons(m, x))) ->
         weight(sum(cons(n, cons(m, x)), cons(0(), x)))
       , weight(cons(n, nil())) -> n}
    StartTerms: basic terms
    Strategy: innermost
  
  Certificate: YES(?,O(n^2))
  
  Proof:
    The weightgap principle applies, where following rules are oriented strictly:
    
    TRS Component:
      {weight(cons(n, cons(m, x))) ->
       weight(sum(cons(n, cons(m, x)), cons(0(), x)))}
    
    Interpretation of nonconstant growth:
    -------------------------------------
      The following argument positions are usable:
        Uargs(sum) = {}, Uargs(cons) = {}, Uargs(s) = {},
        Uargs(weight) = {1}
      We have the following EDA-non-satisfying and IDA(1)-non-satisfying matrix interpretation:
      Interpretation Functions:
       sum(x1, x2) = [0 0] x1 + [1 0] x2 + [1]
                     [0 0]      [0 0]      [1]
       cons(x1, x2) = [0 0] x1 + [0 0] x2 + [0]
                      [1 0]      [0 0]      [3]
       s(x1) = [0 0] x1 + [0]
               [0 0]      [0]
       0() = [0]
             [0]
       nil() = [0]
               [0]
       weight(x1) = [1 1] x1 + [1]
                    [0 0]      [1]
    
    The strictly oriented rules are moved into the weak component.
    
    We consider the following Problem:
    
      Strict Trs:
        {  sum(cons(s(n), x), cons(m, y)) -> sum(cons(n, x), cons(s(m), y))
         , sum(cons(0(), x), y) -> sum(x, y)
         , sum(nil(), y) -> y
         , weight(cons(n, nil())) -> n}
      Weak Trs:
        {weight(cons(n, cons(m, x))) ->
         weight(sum(cons(n, cons(m, x)), cons(0(), x)))}
      StartTerms: basic terms
      Strategy: innermost
    
    Certificate: YES(?,O(n^2))
    
    Proof:
      The weightgap principle applies, where following rules are oriented strictly:
      
      TRS Component:
        {sum(cons(s(n), x), cons(m, y)) -> sum(cons(n, x), cons(s(m), y))}
      
      Interpretation of nonconstant growth:
      -------------------------------------
        The following argument positions are usable:
          Uargs(sum) = {}, Uargs(cons) = {}, Uargs(s) = {},
          Uargs(weight) = {1}
        We have the following EDA-non-satisfying and IDA(1)-non-satisfying matrix interpretation:
        Interpretation Functions:
         sum(x1, x2) = [0 1] x1 + [1 0] x2 + [0]
                       [1 0]      [0 0]      [0]
         cons(x1, x2) = [0 1] x1 + [0 0] x2 + [0]
                        [1 0]      [0 1]      [0]
         s(x1) = [1 0] x1 + [1]
                 [0 1]      [0]
         0() = [0]
               [0]
         nil() = [0]
                 [0]
         weight(x1) = [1 1] x1 + [1]
                      [0 0]      [1]
      
      The strictly oriented rules are moved into the weak component.
      
      We consider the following Problem:
      
        Strict Trs:
          {  sum(cons(0(), x), y) -> sum(x, y)
           , sum(nil(), y) -> y
           , weight(cons(n, nil())) -> n}
        Weak Trs:
          {  sum(cons(s(n), x), cons(m, y)) -> sum(cons(n, x), cons(s(m), y))
           , weight(cons(n, cons(m, x))) ->
             weight(sum(cons(n, cons(m, x)), cons(0(), x)))}
        StartTerms: basic terms
        Strategy: innermost
      
      Certificate: YES(?,O(n^2))
      
      Proof:
        The weightgap principle applies, where following rules are oriented strictly:
        
        TRS Component: {sum(nil(), y) -> y}
        
        Interpretation of nonconstant growth:
        -------------------------------------
          The following argument positions are usable:
            Uargs(sum) = {}, Uargs(cons) = {}, Uargs(s) = {},
            Uargs(weight) = {1}
          We have the following EDA-non-satisfying and IDA(1)-non-satisfying matrix interpretation:
          Interpretation Functions:
           sum(x1, x2) = [0 0] x1 + [1 0] x2 + [1]
                         [0 0]      [0 1]      [1]
           cons(x1, x2) = [1 0] x1 + [0 1] x2 + [0]
                          [0 0]      [0 1]      [1]
           s(x1) = [0 0] x1 + [0]
                   [0 0]      [0]
           0() = [0]
                 [0]
           nil() = [0]
                   [1]
           weight(x1) = [1 0] x1 + [0]
                        [1 0]      [0]
        
        The strictly oriented rules are moved into the weak component.
        
        We consider the following Problem:
        
          Strict Trs:
            {  sum(cons(0(), x), y) -> sum(x, y)
             , weight(cons(n, nil())) -> n}
          Weak Trs:
            {  sum(nil(), y) -> y
             , sum(cons(s(n), x), cons(m, y)) -> sum(cons(n, x), cons(s(m), y))
             , weight(cons(n, cons(m, x))) ->
               weight(sum(cons(n, cons(m, x)), cons(0(), x)))}
          StartTerms: basic terms
          Strategy: innermost
        
        Certificate: YES(?,O(n^2))
        
        Proof:
          The weightgap principle applies, where following rules are oriented strictly:
          
          TRS Component: {weight(cons(n, nil())) -> n}
          
          Interpretation of nonconstant growth:
          -------------------------------------
            The following argument positions are usable:
              Uargs(sum) = {}, Uargs(cons) = {}, Uargs(s) = {},
              Uargs(weight) = {1}
            We have the following EDA-non-satisfying and IDA(1)-non-satisfying matrix interpretation:
            Interpretation Functions:
             sum(x1, x2) = [0 0] x1 + [1 0] x2 + [1]
                           [0 0]      [0 1]      [1]
             cons(x1, x2) = [1 0] x1 + [0 2] x2 + [0]
                            [0 1]      [0 1]      [2]
             s(x1) = [0 0] x1 + [0]
                     [0 0]      [0]
             0() = [3]
                   [0]
             nil() = [0]
                     [1]
             weight(x1) = [1 0] x1 + [0]
                          [0 1]      [0]
          
          The strictly oriented rules are moved into the weak component.
          
          We consider the following Problem:
          
            Strict Trs: {sum(cons(0(), x), y) -> sum(x, y)}
            Weak Trs:
              {  weight(cons(n, nil())) -> n
               , sum(nil(), y) -> y
               , sum(cons(s(n), x), cons(m, y)) -> sum(cons(n, x), cons(s(m), y))
               , weight(cons(n, cons(m, x))) ->
                 weight(sum(cons(n, cons(m, x)), cons(0(), x)))}
            StartTerms: basic terms
            Strategy: innermost
          
          Certificate: YES(?,O(n^2))
          
          Proof:
            We consider the following Problem:
            
              Strict Trs: {sum(cons(0(), x), y) -> sum(x, y)}
              Weak Trs:
                {  weight(cons(n, nil())) -> n
                 , sum(nil(), y) -> y
                 , sum(cons(s(n), x), cons(m, y)) -> sum(cons(n, x), cons(s(m), y))
                 , weight(cons(n, cons(m, x))) ->
                   weight(sum(cons(n, cons(m, x)), cons(0(), x)))}
              StartTerms: basic terms
              Strategy: innermost
            
            Certificate: YES(?,O(n^2))
            
            Proof:
              The following argument positions are usable:
                Uargs(sum) = {}, Uargs(cons) = {}, Uargs(s) = {},
                Uargs(weight) = {1}
              We have the following constructor-based EDA-non-satisfying and IDA(2)-non-satisfying matrix interpretation:
              Interpretation Functions:
               sum(x1, x2) = [0 0 1] x1 + [1 0 0] x2 + [0]
                             [0 0 0]      [0 2 0]      [0]
                             [0 0 0]      [0 0 2]      [1]
               cons(x1, x2) = [1 1 0] x1 + [1 2 0] x2 + [0]
                              [0 0 1]      [0 0 1]      [1]
                              [0 0 0]      [0 0 1]      [1]
               s(x1) = [0 0 0] x1 + [0]
                       [0 0 0]      [0]
                       [0 0 0]      [0]
               0() = [0]
                     [0]
                     [0]
               nil() = [1]
                       [0]
                       [1]
               weight(x1) = [2 0 0] x1 + [0]
                            [1 0 0]      [0]
                            [2 1 0]      [0]

Hurray, we answered YES(?,O(n^2))