Write a CFG $($Context$-$free Grammar$)$ for the following languages.

a$.L=\{a^n{b}^n{c}^m{d}^m$:$n1,m1\}U\{a^n{b}^m{c}^m{d}^n$:$n1,m1\}$

b$.(aUb)**b(aUb)**a(aUb)**$