練習問題 4.2.2

すべての 
xs
について真となる等式はどれか，また偽となるのはどれか． 



[[]] ++ xs   = xs
[[]] ++ xs   = [xs]
[[]] ++ xs   = [[],xs]
[[]] ++ [xs] = [[],xs]
[xs] ++ []   = [xs]
[xs] ++ [xs] = [xs,xs]




常に真なのは


[[]] ++ [xs] = [[],xs]
[xs] ++ []   = [xs]
[xs] ++ [xs] = [xs,xs]


常に偽なのは


[[]] ++ xs   = xs
[[]] ++ xs   = [[],xs]