The prompt |: is displayed instead of the | ?- prompt if your program requires input from the terminal. The built-in predicate prompt/2 can be used to change the form of this prompt.

If you are typing a term at any Prolog prompt, and your input is longer than one line, all lines after the first one are indented five spaces. Sometimes this arises unexpectedly because of a typing error. For example, if you type

     | ?- f('ABC).

you will see your cursor positioned where the underscore character appears here. This signifies that you have not completed the input of a term: in this case there was no closing quote. To get back to the top level prompt type a closing quote followed by a period and a <RET>. This will give a syntax error after which you can type the correct goal.