Question: error in simplify

> coth;
                                    coth
> restart;
> c := 0;
                                      0
> w := -2*mu;
                                    -2 mu
> a[-1] := 0;
                                      0
> a[0] := mu*lambda*sqrt(-6*a);
                                            (1/2)
                            mu lambda (-6 a)     
> a[1] := (6*(mu*lambda^2+1))/sqrt(-6*a);
                               /         2    \
                             6 \mu lambda  + 1/
                             ------------------
                                      (1/2)    
                                (-6 a)         
> b[-1] := 0;
                                      0
> b[0] := 0;
                                      0
> b[1] := 0;
                                      0
> xi := x+w*t;
                                 x - 2 mu t
> P := sqrt(-mu)*coth(A+sqrt(-mu)*xi);
                     (1/2)     /         (1/2)             \
                (-mu)      coth\A + (-mu)      (x - 2 mu t)/
> u := a[0]+a[1]*P/(1+lambda*P)+a[-1]*(1+lambda*P)/P+b[0]*sqrt(sigma*(1+P^2/mu))/P+b[1]*sqrt(sigma*(1+P^2/mu))+b[-1]*sqrt(sigma*(1+P^2/mu))/P^2;
                 (1/2)
 mu lambda (-6 a)     

           /         2    \      (1/2)     /         (1/2)             \   
         6 \mu lambda  + 1/ (-mu)      coth\A + (-mu)      (x - 2 mu t)/   
    + ---------------------------------------------------------------------
            (1/2) /                (1/2)     /         (1/2)             \\
      (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//
> Diff(u, t)+a*u^2*(Diff(u, x))+Diff(u, `$`(x, 3));
/    /                     
| d  |                (1/2)
|--- |mu lambda (-6 a)     
| dt |                     
\    \                     

          /         2    \      (1/2)     /         (1/2)             \   \\     /          
        6 \mu lambda  + 1/ (-mu)      coth\A + (-mu)      (x - 2 mu t)/   ||     |          
   + ---------------------------------------------------------------------|| + a |mu lambda
           (1/2) /                (1/2)     /         (1/2)             \\||     |          
     (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)////     \          

        (1/2)
  (-6 a)     

          /         2    \      (1/2)     /         (1/2)             \   \   
        6 \mu lambda  + 1/ (-mu)      coth\A + (-mu)      (x - 2 mu t)/   |   
   + ---------------------------------------------------------------------|^2
           (1/2) /                (1/2)     /         (1/2)             \\|   
     (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)///   

  /    /                     
  | d  |                (1/2)
  |--- |mu lambda (-6 a)     
  | dx |                     
  \    \                     

          /         2    \      (1/2)     /         (1/2)             \   \\   
        6 \mu lambda  + 1/ (-mu)      coth\A + (-mu)      (x - 2 mu t)/   ||   
   + ---------------------------------------------------------------------|| +
           (1/2) /                (1/2)     /         (1/2)             \\||   
     (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)////   

  / 3 /                     
  |d  |                (1/2)
  |-- |mu lambda (-6 a)     
  |   |                     
  \   \                     

          /         2    \      (1/2)     /         (1/2)             \   \\
        6 \mu lambda  + 1/ (-mu)      coth\A + (-mu)      (x - 2 mu t)/   ||
   + ---------------------------------------------------------------------||
           (1/2) /                (1/2)     /         (1/2)             \\||
     (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)////
> value(%);
                          /                                     2\      
     /         2    \   2 |        /         (1/2)             \ |      
  12 \mu lambda  + 1/ mu  \1 - coth\A + (-mu)      (x - 2 mu t)/ /      
--------------------------------------------------------------------- -
      (1/2) /                (1/2)     /         (1/2)             \\   
(-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//   

                                                                         /   
                                    1                                    |   
  ---------------------------------------------------------------------- \12
                                                                       2     
        (1/2) /                (1/2)     /         (1/2)             \\      
  (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//      

                                                                           /
  /         2    \      (1/2)     /         (1/2)             \          2 |
  \mu lambda  + 1/ (-mu)      coth\A + (-mu)      (x - 2 mu t)/ lambda mu  \1

                                      2\\     /                     
         /         (1/2)             \ ||     |                (1/2)
   - coth\A + (-mu)      (x - 2 mu t)/ // + a |mu lambda (-6 a)     
                                              |                     
                                              \                     

          /         2    \      (1/2)     /         (1/2)             \   \   
        6 \mu lambda  + 1/ (-mu)      coth\A + (-mu)      (x - 2 mu t)/   |   
   + ---------------------------------------------------------------------|^2
           (1/2) /                (1/2)     /         (1/2)             \\|   
     (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)///   

  /                           /                                     2\       
  |       /         2    \    |        /         (1/2)             \ |       
  |     6 \mu lambda  + 1/ mu \1 - coth\A + (-mu)      (x - 2 mu t)/ /       
  |- --------------------------------------------------------------------- +
  |        (1/2) /                (1/2)     /         (1/2)             \\   
  |  (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//   
  \                                                                          

                                                                         /      
                                    1                                    |  /   
  ---------------------------------------------------------------------- \6 \mu
                                                                       2        
        (1/2) /                (1/2)     /         (1/2)             \\         
  (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//         

                                                                      /
        2    \      (1/2)     /         (1/2)             \           |
  lambda  + 1/ (-mu)      coth\A + (-mu)      (x - 2 mu t)/ lambda mu \1

                                         \
                                      2\\|
         /         (1/2)             \ |||
   - coth\A + (-mu)      (x - 2 mu t)/ //|
                                         |
                                         |
                                         /

                                                                       2     
                               /                                     2\      
          /         2    \   2 |        /         (1/2)             \ |      
       12 \mu lambda  + 1/ mu  \1 - coth\A + (-mu)      (x - 2 mu t)/ /      
   - --------------------------------------------------------------------- +
           (1/2) /                (1/2)     /         (1/2)             \\   
     (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//   

                                                                        /       
                                    1                                   |   /   
  --------------------------------------------------------------------- \24 \mu
        (1/2) /                (1/2)     /         (1/2)             \\         
  (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//         

                                                    2 /
        2    \   2     /         (1/2)             \  |
  lambda  + 1/ mu  coth\A + (-mu)      (x - 2 mu t)/  \1

                                      2\\   
         /         (1/2)             \ ||   
   - coth\A + (-mu)      (x - 2 mu t)/ // +

                                                                         /   
                                                                         |   
                                    1                                    |   
  ---------------------------------------------------------------------- \84
                                                                       2     
        (1/2) /                (1/2)     /         (1/2)             \\      
  (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//      

  /         2    \   2     /         (1/2)             \
  \mu lambda  + 1/ mu  coth\A + (-mu)      (x - 2 mu t)/

                                          2                  \
  /                                     2\                   |
  |        /         (1/2)             \ |       (1/2)       |
  \1 - coth\A + (-mu)      (x - 2 mu t)/ /  (-mu)      lambda/

                                                                     3           
                             /                                     2\            
        /         2    \   3 |        /         (1/2)             \ |        2   
     36 \mu lambda  + 1/ mu  \1 - coth\A + (-mu)      (x - 2 mu t)/ /  lambda    
   - ------------------------------------------------------------------------- +
                                                                           3     
            (1/2) /                (1/2)     /         (1/2)             \\      
      (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//      

                                                                         /   
                                                                         |   
                                    1                                    |   
  ---------------------------------------------------------------------- \36
                                                                       4     
        (1/2) /                (1/2)     /         (1/2)             \\      
  (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//      

  /         2    \      (1/2)     /         (1/2)             \       3   3
  \mu lambda  + 1/ (-mu)      coth\A + (-mu)      (x - 2 mu t)/ lambda  mu  

                                          3\   
  /                                     2\ |   
  |        /         (1/2)             \ | |   
  \1 - coth\A + (-mu)      (x - 2 mu t)/ / / +

                                                                         /   
                                                                         |   
                                    1                                    |   
  ---------------------------------------------------------------------- \72
                                                                       3     
        (1/2) /                (1/2)     /         (1/2)             \\      
  (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//      

                                                        2         
  /         2    \   3     /         (1/2)             \        2
  \mu lambda  + 1/ mu  coth\A + (-mu)      (x - 2 mu t)/  lambda  

                                          2\   
  /                                     2\ |   
  |        /         (1/2)             \ | |   
  \1 - coth\A + (-mu)      (x - 2 mu t)/ / / -

                                                                         /   
                                    1                                    |   
  ---------------------------------------------------------------------- \24
                                                                       2     
        (1/2) /                (1/2)     /         (1/2)             \\      
  (-6 a)      \1 + lambda (-mu)      coth\A + (-mu)      (x - 2 mu t)//      

                                                        3        /
  /         2    \   2     /         (1/2)             \         |
  \mu lambda  + 1/ mu  coth\A + (-mu)      (x - 2 mu t)/  lambda \1

                                      2\           \
         /         (1/2)             \ |      (1/2)|
   - coth\A + (-mu)      (x - 2 mu t)/ / (-mu)     /
> simplify(%);
Error, (in simplify/tools/_zn) too many levels of recursion
>
>
>
>
pls help