@Kitonum 

And in order to make it a list, is this a good way(?):

[op({L1[], L2[]})];

@Carl Love 

Thanks; Very useful, I appreciate it.

Not exactly the same problem, but soemthing similar:

Having two list of monomials, say

L1:=[y,x*y];
L2:=[x^2,x*y];

I'd like to have

L:=[y,x*y,x^2];

For that I do

L:= [{op(L1)} union {op(L2)} ];

But then according to what you said this shouldn't be efficient. (Or am I missing something?) What would you suggest for that?

@Carl Love @Kitonum 9480

Thanks!

Hi Again @MapleMathMatt 

I updated the package (I'm using Maple 18), but the error persists!

Thanks everynody for the interesting idea :)

Bur I get this arror:

Error, (in mondeg) invalid input: add expects 2 arguments, but received 1

Any idea?

@MapleMathMatt 

Thanks; Copy-pasting exactly your code, I get this error though:

Error, (in Physics:-Library:-Add) unable to determine the dummy summation variable; expected a name, received: j

Any idea?