public final Expr make(ItemList list)
{
ItemList x = list.cdr(); // drop "sum"
boolean distinct = startsWithDistinct(x) ;
if ( distinct )
x = x.cdr();
BuilderLib.checkLength(1, x, "Broken syntax: "+list.shortString()) ;
// (sum ?var)
Expr expr = buildExpr(x.get(0)) ;
return make(distinct, expr) ;
}