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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 _sat_formula(lexer, varname, fmt, nvars):
 """Return a DIMACS SAT formula."""

    types = {IntegerToken, LPAREN} | _SAT_TOKS[fmt]
    tok = _expect_token(lexer, types)
    # INT
    if isinstance(tok, IntegerToken):
        index = tok.value
        if not 0 < index <= nvars:
            fstr = "formula literal {} outside valid range: (0, {}]"
            raise Error(fstr.format(index, nvars))
        return ('var', (varname, ), (index, ))
    # '-'
    elif isinstance(tok, OP_not):
        tok = _expect_token(lexer, {IntegerToken, LPAREN})
        # '-' INT
        if isinstance(tok, IntegerToken):
            index = tok.value
            if not 0 < index <= nvars:
                fstr = "formula literal {} outside valid range: (0, {}]"
                raise Error(fstr.format(index, nvars))
            return ('not', ('var', (varname, ), (index, )))
        # '-' '(' FORMULA ')'
        else:
            formula = _sat_formula(lexer, varname, fmt, nvars)
            _expect_token(lexer, {RPAREN})
            return ('not', formula)
    # '(' FORMULA ')'
    elif isinstance(tok, LPAREN):
        formula = _sat_formula(lexer, varname, fmt, nvars)
        _expect_token(lexer, {RPAREN})
        return formula
    # OP '(' FORMULAS ')'
    else:
        _expect_token(lexer, {LPAREN})
        formulas = _formulas(lexer, varname, fmt, nvars)
        _expect_token(lexer, {RPAREN})
        return (tok.ASTOP, ) + formulas