代码上的一处错误

[JiajunDu]

C08/LogicCalculator.hs代码中，第57行的函数体有一处错误，可以尝试以下的测试样例：

ex5 :: Formula (Int, ())
ex5 = Exist [1, 2, 3] $ \n -> Body (n :=: Con 3)
solutions ex5

代码应修改为：

solutions (Exist (x:xs) as) = [(x,fs) | fs <- solutions (as (Con x)),
                                            satisfiable (as (Con x))] 
                              ++ solutions (Exist xs as)