happy(X) :- rich(X), famous(X).

says that X is happy if X is rich and X is famous. What about logical-or, ∨, also called disjunction? If we want to say that X is happy if X is richor if X is famous, we have two possibilities:

1. 
   use two alternative rules


2. 
   use the ;operator (pronounced "or").

Option 1:

happy1(X) :- rich(X).
happy1(X) :- famous(X).

Option 2:

happy2(X) :- rich(X) ; famous(X).

Sometimes it is necessary, or at least advisable, to wrap parentheses ( ) around a disjunction in order to make clear what how the conjunctions and disjunctions interact. If you leave out the parentheses, Prolog has its own rules about the precedence of logical-and and logical-or, which might or might not correspond to what you intend. If you use parentheses, the things inside the parentheses are done first, so everything is crystal clear.

Option 3:

happy3(X) :- attractive(X), ( rich(X) ; famous(X) ).

This says X is happy3 if X is attractive, and X is either rich or famous:

happy3(X) ⇐ attractive(X) ∧ (rich(X) ∨ famous(X)).*

* The Prolog Dictionary wishes to acknowledge that a range of world religions and philosophies would not agree that the criteria specified for happy1, happy2, and happy3 are either necessary nor sufficient for happiness. The definitions given in this article, are, therefore, purely intended as Prolog programming examples, and should not be taken as a practical guide to life. Thank you.

• 
  write(X) which writes the term X to the current outputstream (which means the window on your workstation unless you have done something fancy).


• 
  print(X, …) which writes a variable number of arguments to the current output stream. If an argument is a string (like 'Hello world!n') then the string is printed without quotes, and any 'n' and 't' are interpreted as newline and tab, respectively. A newline is printed at the end of the printing operation.


• 
  prin(X, …) is like print except that it does not append a newline to the end of the output.


• 
  tab(N) prints N spaces to output - N should be a number.


• 
  nl starts a new line.


• 
  put(C) prints a single character to the current output stream. C might be bound to a number in the range 0 to 255, in which case the number is treated as a character code, and the equivalentcharacter is printed, or it might be bound to a character, e.g. C = 'a', in which case obviously the character is printed.