[NTG-context] Substack in Context

Hi, What is the context equivalent of latex's substack? How do I type the following $$ \sum_{\substack i \in P \\ j \in Q} a_{ij} $$ I search on source browser shows that this command was defined in newmat but pruned out. Is there a replacement for it? Is it safe to simply copy the definitions from m-newmat.tex and use them in my project? Thanks Aditya -- Aditya Mahajan, EECS Systems, University of Michigan http://www.eecs.umich.edu/~adityam || Ph: 7342624008