implemented some set operations
implemented some set operations

svn path=/trunk/StrategoXT/; revision=1570

Apanatshka/strategoxt,Apanatshka/strategoxt,lichtemo/strategoxt,Apanatshka/strategoxt,Apanatshka/strategoxt,metaborg/strategoxt,metaborg/strategoxt,metaborg/strategoxt,lichtemo/strategoxt,lichtemo/strategoxt,lichtemo/strategoxt,lichtemo/strategoxt,metaborg/strategoxt,metaborg/strategoxt,Apanatshka/strategoxt