static L collectNonNulls(Collection c, S field) {
  ret collectNonNulls(field, c);
}

static L mapMethodLike collectNonNulls(S field, Collection c) {
  new L l;
  if (c != null) for (O a : c) {
    O x = getOpt(a, field);
    if (x != null) l.add(x);
  }
  ret l;
}

/*ifclass Concept
static L collect(Class c, S field) {
  ret collect(list(c), field);
}
endif
TODO: make translator ignore stuff in ifclass until resolved
*/