[stack] Is this language concatenative?

Hi!

Consider the following example. Is this language concatenative?

The set of  terminal symbols T = { 'a', '(', ')' }.

The grammar G_0 has the following production rules:
S  -> e               # empty string
S  -> S S
S  -> ( S1 )
S1 -> a | a S1

The grammar G_0 generates the language L \subset T*.
Some elements of L are
e, (a), (a)(aa), (aaa)(a), etc.

We have LL = L, i.e. the concatenation of two elements of L belongs to L.

It seems to me that L is a concatenative language, but it doesn't satisfy
the Christopher's definition (there is no concatenative grammar which 
generates this language - the proof is a simple exercise).

What do you think?

Michael Nedzelsky