Parsed program:

parent(abraham,ishmael).
parent(abraham,isaac).
parent(isaac,esau).
parent(isaac,jacob).
grandparent(X,Y) :- parent(X,Z),parent(Z,Y).
descendant(X,Y) :- parent(Y,X).
descendant(X,Y) :- parent(Z,X),descendant(Z,Y).

Parsed query: 
grandparent(X,Y).

Entering BC
Current goal: grandparent(X,Y).
Additional goals: 0
Unified goal grandparent(X,Y). and clause grandparent(X1,Y1) :- parent(X1,Z1),parent(Z1,Y1).
Substitution:
    'X1'    'X'
    'Y1'    'Y'


Entering BC
Current goal: parent(X,Z1).
Additional goals: 1
Unified goal parent(X,Z1). and clause parent(abraham,ishmael).
Substitution:
    'X'     'abraham'
    'Z1'    'ishmael'


Entering BC
Current goal: parent(ishmael,Y).
Additional goals: 0
Exiting BC
Unified goal parent(X,Z1). and clause parent(abraham,isaac).
Substitution:
    'X'     'abraham'
    'Z1'    'isaac'  


Entering BC
Current goal: parent(isaac,Y).
Additional goals: 0
Unified goal parent(isaac,Y). and clause parent(isaac,esau).
Substitution:
    'Y'    'esau'


Entering BC
Found answer
    'X1'    'X'      
    'Y1'    'Y'      
    'X'     'abraham'
    'Z1'    'isaac'  
    'Y'     'esau'   

Unified goal parent(isaac,Y). and clause parent(isaac,jacob).
Substitution:
    'Y'    'jacob'


Entering BC
Found answer
    'X1'    'X'      
    'Y1'    'Y'      
    'X'     'abraham'
    'Z1'    'isaac'  
    'Y'     'jacob'  

Exiting BC
Unified goal parent(X,Z1). and clause parent(isaac,esau).
Substitution:
    'X'     'isaac'
    'Z1'    'esau' 


Entering BC
Current goal: parent(esau,Y).
Additional goals: 0
Exiting BC
Unified goal parent(X,Z1). and clause parent(isaac,jacob).
Substitution:
    'X'     'isaac'
    'Z1'    'jacob'


Entering BC
Current goal: parent(jacob,Y).
Additional goals: 0
Exiting BC
Exiting BC
Exiting BC

Query: grandparent(X,Y).
Yes

answer = 

    'X'    'abraham'
    'Y'    'esau'   


answer = 

    'X'    'abraham'
    'Y'    'jacob'