code: public void add(ColumnFamily columnFamily)
      {
          assert columnFamily != null;
          ColumnFamily prev = modifications.put(columnFamily.id(), columnFamily);
          if (prev != null)
              // developer error
              throw new IllegalArgumentException("ColumnFamily " + columnFamily + " already has modifications in this mutation: " + prev);
      }
docs: /*
  Specify a column family name and the corresponding column
  family object.
  param @ cf - column family name
  param @ columnFamily - the column family.