public T GetValue <T>(AssignmentField <T> field)
 {
     return((T)GetValue((AssignmentField)field));
 }
 public Assignment SetValue <T>(AssignmentField <T> field, T value)
 {
     return(SetValue((AssignmentField)field, value));
 }
 public bool HasValue(AssignmentField field)
 {
     return(_data.HasValue(field));
 }