Slide 14.17: Facts with arguments
Slide 14.19: Variables and unification
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Facts with Arguments (Cont.)

Examples of Facts with Arguments (Cont.) 
   eats(fred,oranges).        /* Fred eats oranges. */
   eats(fred,t_bone_steaks).  /* Fred eats T-bone steaks. */
   eats(tony,apples).         /* Tony eats apples. */
   eats(john,apples).         /* John eats apples. */
   eats(john,grapefruit).     /* John eats grapefruit. */
 ?- eats(mike,apples).   /* Does mike eat apples? */

 no                      /* No, according to the database */

 ?- eats(fred,apples).   /* Does fred eat apples? */

 no                      /* No, we don't know. */
Let us consider another database. This time we will use the predicate age to indicate the ages of various individuals. 
   age( john, 32 ).             /* John is 32 years old. */
   age( agnes, 41 ).            /* Agnes is 41. */
   age( george, 72 ).           /* George is 72. */
   age( ian, 2 ).               /* Ian is 2. */
   age( thomas, 25 ).           /* Thomas is 25. */
If we now ask some queries we would get the following interaction: 
 ?- age( ian, 2 ).  /* Is Ian 2 years old? */

 yes                /* Yes, the fourth clause */

 ?- agnes( 41 ).    /* Is agnes 41? */

 no                 /* No. In the database we only */
                    /* know about the relation age, */
                    /* not about the relation agnes, */
                    /* so the query fails. */

 ?- age(ian,two).   /* Is Ian two years old? */

 no                 /* No. two and 2 are not the same. */
                    /* Therefore don't match. */