Represent the following problem using first-order logic sentences, convert the sentences to conjunctive normal form (CNF), and use resolution proof by refutation following the procedure described here " For each resolution step, give the numbers of the two clauses to be resolved, show the resulting clause, and give it a new number. If the clauses involve variables, then standardize apart the variables and show the substitution list for these variables"

All men are mortal. There exists a man. Prove there exists a mortal.