I'm trying to solve this problem for my class. But now I'm so confused about the following question. 

\*Consider the following statements:

1. Every boy or girl is a child.
2. Every child gets a doll or a train or a lump of coal.
3. No boy gets a doll.
4. No child who is good gets a lump of coal.

a. Use the predicates BOY, GIRL, CHILD, GET-DOLL, GET-TRAIN, GET-COAL and GOOD to represent these statements as predicate calculus formulas. 

b. Convert these statements to clauses. 

c. Use resolution refutation to prove that if no child gets a train, then no boy is good.\* 

after first two-step, I can't prove the conclusion. One reason I think is that there is a girl function in the first clause, so I can't get an empty clause finally. The second reason is that I'm not sure whether I can use a clause many times to do the resolution refutation. Other similar questions are easy to prove by just using clause once and join them together, finally, I can get NIL or empty clause. But I can't prove this one. I don't know how to do.

[The working step 01](https://i.stack.imgur.com/SLEXf.png)

[The working step 02(Please ignore this, I think the goal is wrong in this image](https://i.stack.imgur.com/H7jn2.png)

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[Goal and resolution refutation - 1(New version)](https://i.stack.imgur.com/g165D.png)

[Goal and resolution refutation - 2(New version)](https://i.stack.imgur.com/pLrtG.png)