It is well known that a program—to accomplish any task—is welcomed by new users as long as it is easy to use, to understand, to learn. In the recent past, programs were written first and a nice adhoc interface was added later. The look and feel of this interface gradually became the convincing part of the whole system (program plus interface): in fact the way in which the screen presented the available commands, the possibility of on-line help, of undo operations, etc. were firstly greeted and then requested by the new users of complex programs.