Why f(xi, y), instead of f(xi,xj,y) or f(xi,xj,xk,y)?

From the implementation, only f(xi, y) is considered as features, It basically discards the correlations among variables. Is it the general case within MEM to ignore f(xi,xj, y) or f(xi, xj, xk, y)?
```
for (int i = 1; i < segs.length; ++i){
fieldList.add(segs[i]);
Feature feature = new Feature(label, segs[i]);
int index = featureList.indexOf(feature);
if (index == -1)
{
featureList.add(feature);
featureCountList.add(1);
}
else
{
featureCountList.set(index, featureCountList.get(index) + 1);
}
}
```