/* 1  */ color(banana, yellow).
/* 2  */ color(squash, yellow).
/* 3  */ color(apple, green).
/* 4  */ color(peas, green).
/* 5  */ color(broccoli, green).
/* 6  */ fruit(banana).
/* 7  */ fruit(apple).
/* 8  */ vegetable(squash).
/* 9  */ vegetable(peas).
/* 10 */ vegetable(broccoli).

/* 11 */ eats(bob, X) :- color(X, green), vegetable(X).

And a query: eats(bob, Y).

• Goal: eats(bob, Y) in other words, what does bob eat?
• MATCH 1: Clause 11 Instantiate: {X=Y}

• Goal: color(Y, green), vegetable(Y)
• MATCH 2: Clause 3 Instantiate: {X=Y=apple}

• Goal: vegetable(apple)
• Fail. Backtrack and redo MATCH 2, starting at clause 4

• Goal: color(Y, green), vegetable(Y)
• MATCH 3: Clause 4 Instantiate: {X=Y=peas}

• Goal: vegetable(peas)
• MATCH 4: Clause 9
• Success: Y=peas. eats(bob, peas)!

• User types ";". Backtrack. Redo MATCH 4, starting at clause 10

• Goal: vegetable(peas)
• Fail. Backtrack and redo MATCH 3, starting at clause 5

• Goal: color(Y, green), vegetable(Y)
• MATCH 5: Clause 5 Instantiate: {X=Y=broccoli}

• Goal: vegetable(broccoli)
• MATCH 6: Clause 10
• Success: Y=broccoli. eats(bob, broccoli)!

• User types ";". Backtrack. Redo MATCH 6, starting at clause 11

• Goal: vegetable(broccoli)
• Fail. Backtrack and redo MATCH 5, starting at clause 6

• Goal: color(Y, green), vegetable(Y)
• Fail. Backtrack and redo MATCH 1, starting after clause 11

• Goal: eats(bob, Y)
• Fail.