Remove interactive mode, replace it with additional help option.
The interactive mode is presumably not used by anyone. I propose therefore to remove it and add instead an option to print the help for commands on the command line.
Additionally I noticed that Opal crashes while printing the help messages. The reason is that the type names predefined string, upper case string and upper case string array are longer than 16 characters. However the user doesn't have to know what these types represent, they are simply strings and string arrays respectively.