// Create if does not exist.
if ( accs == null )
{
for ( ExprAggregator agg : aggregators )
{
Accumulator x = agg.getAggregator().createAccumulator() ;
Var v = agg.getVar() ;
accumulators.put(key, Pair.create(v, x)) ;
}
accs = accumulators.get(key) ;
}