Before revision:  In the second part of the paper, we have refined our analysis by considering subsets of nodes regarded in the literature as sharing similar economic traits: while the evolution of certain subgroups of countries and products confirms the trends highlighted by the global motifs, other groupings show a behavior compatible with our null model throughout the whole period 1995-2010, thus questioning the economic relevance traditionally assigned to these groups .
After revision:  We have also refined our analysis by considering subsets of nodes regarded in the literature as sharing similar economic traits: while the evolution of certain subgroups of countries and products confirms the trends highlighted by the global motifs, other groupings show a behavior compatible with our null model throughout the whole period 1995-2010, thus questioning the economic relevance traditionally assigned to these groups .