code: def _cnf_formula(lexer, varname, nvars, nclauses):
      
      clauses = _clauses(lexer, varname, nvars)
  
      if len(clauses) < nclauses:
          fstr = "formula has fewer than {} clauses"
          raise Error(fstr.format(nclauses))
      if len(clauses) > nclauses:
          fstr = "formula has more than {} clauses"
          raise Error(fstr.format(nclauses))
  
      return ('and', ) + clauses
docs: Return a DIMACS CNF formula.