YES(?,O(n^1))
* Step 1: TrivialSCCs WORST_CASE(?,O(n^1))
    + Considered Problem:
        Rules:
          0. f2(A,B,C)   -> f2(-1 + A,B,C)   [A >= 31] (?,1)
          1. f2(A,B,C)   -> f300(A,-1 + B,C) [30 >= A] (?,1)
          2. f300(A,B,C) -> f2(A,B,C)        [B >= 21] (?,1)
          3. f300(A,B,C) -> f1(A,B,D)        [20 >= B] (?,1)
          4. f3(A,B,C)   -> f300(A,B,C)      True      (1,1)
        Signature:
          {(f1,3);(f2,3);(f3,3);(f300,3)}
        Flow Graph:
          [0->{0,1},1->{2,3},2->{0,1},3->{},4->{2,3}]
        
    + Applied Processor:
        TrivialSCCs
    + Details:
        All trivial SCCs of the transition graph admit timebound 1.
* Step 2: PolyRank WORST_CASE(?,O(n^1))
    + Considered Problem:
        Rules:
          0. f2(A,B,C)   -> f2(-1 + A,B,C)   [A >= 31] (?,1)
          1. f2(A,B,C)   -> f300(A,-1 + B,C) [30 >= A] (?,1)
          2. f300(A,B,C) -> f2(A,B,C)        [B >= 21] (?,1)
          3. f300(A,B,C) -> f1(A,B,D)        [20 >= B] (1,1)
          4. f3(A,B,C)   -> f300(A,B,C)      True      (1,1)
        Signature:
          {(f1,3);(f2,3);(f3,3);(f300,3)}
        Flow Graph:
          [0->{0,1},1->{2,3},2->{0,1},3->{},4->{2,3}]
        
    + Applied Processor:
        PolyRank {useFarkas = True, withSizebounds = [], shape = Linear}
    + Details:
        We apply a polynomial interpretation of shape linear:
            p(f1) = -20 + x2
            p(f2) = -21 + x2
            p(f3) = -20 + x2
          p(f300) = -20 + x2
        
        Following rules are strictly oriented:
            [B >= 21] ==>          
          f300(A,B,C)   = -20 + B  
                        > -21 + B  
                        = f2(A,B,C)
        
        
        Following rules are weakly oriented:
            [A >= 31] ==>                 
            f2(A,B,C)   = -21 + B         
                       >= -21 + B         
                        = f2(-1 + A,B,C)  
        
            [30 >= A] ==>                 
            f2(A,B,C)   = -21 + B         
                       >= -21 + B         
                        = f300(A,-1 + B,C)
        
            [20 >= B] ==>                 
          f300(A,B,C)   = -20 + B         
                       >= -20 + B         
                        = f1(A,B,D)       
        
                 True ==>                 
            f3(A,B,C)   = -20 + B         
                       >= -20 + B         
                        = f300(A,B,C)     
        
        
* Step 3: PolyRank WORST_CASE(?,O(n^1))
    + Considered Problem:
        Rules:
          0. f2(A,B,C)   -> f2(-1 + A,B,C)   [A >= 31] (?,1)     
          1. f2(A,B,C)   -> f300(A,-1 + B,C) [30 >= A] (?,1)     
          2. f300(A,B,C) -> f2(A,B,C)        [B >= 21] (20 + B,1)
          3. f300(A,B,C) -> f1(A,B,D)        [20 >= B] (1,1)     
          4. f3(A,B,C)   -> f300(A,B,C)      True      (1,1)     
        Signature:
          {(f1,3);(f2,3);(f3,3);(f300,3)}
        Flow Graph:
          [0->{0,1},1->{2,3},2->{0,1},3->{},4->{2,3}]
        
    + Applied Processor:
        PolyRank {useFarkas = True, withSizebounds = [], shape = Linear}
    + Details:
        We apply a polynomial interpretation of shape linear:
            p(f1) = 1 + x1
            p(f2) = 1 + x1
            p(f3) = 1 + x1
          p(f300) = 1 + x1
        
        Following rules are strictly oriented:
          [A >= 31] ==>               
          f2(A,B,C)   = 1 + A         
                      > A             
                      = f2(-1 + A,B,C)
        
        
        Following rules are weakly oriented:
            [30 >= A] ==>                 
            f2(A,B,C)   = 1 + A           
                       >= 1 + A           
                        = f300(A,-1 + B,C)
        
            [B >= 21] ==>                 
          f300(A,B,C)   = 1 + A           
                       >= 1 + A           
                        = f2(A,B,C)       
        
            [20 >= B] ==>                 
          f300(A,B,C)   = 1 + A           
                       >= 1 + A           
                        = f1(A,B,D)       
        
                 True ==>                 
            f3(A,B,C)   = 1 + A           
                       >= 1 + A           
                        = f300(A,B,C)     
        
        
* Step 4: KnowledgePropagation WORST_CASE(?,O(n^1))
    + Considered Problem:
        Rules:
          0. f2(A,B,C)   -> f2(-1 + A,B,C)   [A >= 31] (1 + A,1) 
          1. f2(A,B,C)   -> f300(A,-1 + B,C) [30 >= A] (?,1)     
          2. f300(A,B,C) -> f2(A,B,C)        [B >= 21] (20 + B,1)
          3. f300(A,B,C) -> f1(A,B,D)        [20 >= B] (1,1)     
          4. f3(A,B,C)   -> f300(A,B,C)      True      (1,1)     
        Signature:
          {(f1,3);(f2,3);(f3,3);(f300,3)}
        Flow Graph:
          [0->{0,1},1->{2,3},2->{0,1},3->{},4->{2,3}]
        
    + Applied Processor:
        KnowledgePropagation
    + Details:
        We propagate bounds from predecessors.
* Step 5: PolyRank WORST_CASE(?,O(n^1))
    + Considered Problem:
        Rules:
          0. f2(A,B,C)   -> f2(-1 + A,B,C)   [A >= 31] (1 + A,1)     
          1. f2(A,B,C)   -> f300(A,-1 + B,C) [30 >= A] (21 + A + B,1)
          2. f300(A,B,C) -> f2(A,B,C)        [B >= 21] (20 + B,1)    
          3. f300(A,B,C) -> f1(A,B,D)        [20 >= B] (1,1)         
          4. f3(A,B,C)   -> f300(A,B,C)      True      (1,1)         
        Signature:
          {(f1,3);(f2,3);(f3,3);(f300,3)}
        Flow Graph:
          [0->{0,1},1->{2,3},2->{0,1},3->{},4->{2,3}]
        
    + Applied Processor:
        PolyRank {useFarkas = True, withSizebounds = [], shape = Linear}
    + Details:
        The problem is already solved.

YES(?,O(n^1))