code: def _cnf(lexer, varname):
      
      _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)
docs: Return a DIMACS CNF.