On: Symbolica Computer Algebra System

[SPACECADET3D]

It's not a language specific feature, but you can do the bookkeeping yourself. In the simplest form you can write:

    f(1,...)*f(2,...)

Then when you do pattern matching with wildcards x_ and y_ you can do

   f(x_,...)*f(y_,..)

with the requirement that x_ < y_. This way you will know you match them in the expected order and not the other way around.