Question: use of implicit plot

I have written the code below, and received the error message in the bottom. Can any body help me?


> with(plots);

> u := 100*sin((1/2)*pi*x)*((1-cosh(pi))*sinh((1/2)*pi*y)+sinh(pi)*cosh((1/2)*pi*y))/sin(pi);

> implicitplot({u = 10, u = 20, u = 30, u = 40, u = 50, u = 60, u = 70, u = 80, u = 90}, x = 0 .. 2, y = 0 .. 2, grid = [100, 100]);

Error, (in plots/implicitplot) invalid input: the following extra unknowns were found in the input expression: {pi}
>

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