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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _cnf(lexer, varname):
 """Return a DIMACS CNF."""

    _expect_token(lexer, {KW_p})
    _expect_token(lexer, {KW_cnf})
    nvars = _expect_token(lexer, {IntegerToken}).value
    nclauses = _expect_token(lexer, {IntegerToken}).value
    return _cnf_formula(lexer, varname, nvars, nclauses)