練習問題 4.2.1
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]常に真なのは
xs:[] = [xs]上記以外は偽
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]常に真なのは
xs:[] = [xs]上記以外は偽