code: @Deprecated
    public final Assignment intersection(Collection<Integer> varNums) {
      return intersection(Ints.toArray(varNums));
    }
docs: If varNums is a subset of the variables in this assignment, this method
  returns the value assigned to each variable in varNums. varNums may contain
  indices which are not represented in this assignment. These indices are
  ignored; in this case, the returned assignment will contain fewer
  variable/value mappings than {@code varNums.size()}.