refere to http://www.maplesoft.com/support/help/Maple/view.aspx?path=PDEtools/SymmetryTest

1. What is the purpose of using SymmetryTest for PDE?

2. What is the purpose of using infinitesimals?

3. What is the purpose of using InfinitesimalsGenerator?

4. Any documents or books mentioning above in detail?

i use maple 12

with(PDETools):

U := diff_table(u(x,y)):

declare(U)

error, (in PDEtools:-declare) cannot declare objects as U

which package can calculate schwinger function?

How to skip error and continue to run in procedure?

> with(DifferentialAlgebra:-Tools);
Error, `DifferentialAlgebra` does not evaluate to a module
with(DifferentialAlgebra):
> with(DifferentialAlgebra);
Error, invalid input: with expects its 1st argument, pname, to be of type {`module`, package}, but received DifferentialAlgebra

don't know why maple 12 give these error, how to import differentialalgebra package?