def letter$\_$count$($alphabet: list$[$list$[$str$]],$

letter: str$)->$ int:

$"""$Return the number of occurrences of letter in alphabet.

$>>>$ alphabet $=[[\text{'}$a$\text{'},\text{'}$b$\text{'},\text{'}$c$\text{'}],[\text{'}$b$\text{'},\text{'}$c$\text{'},\text{'}$d$\text{'}],[\text{'}$d$\text{'},\text{'}$b$\text{'},\text{'}$e$\text{'}]]$

$>>>$ letter$\_$count$($grid$,\text{'}$b$\text{'})$

$3$

$>>>$ alphabet $=[[\text{'}$d$\text{'},\text{'}$e$\text{'},\text{'}$f$\text{'}],[\text{'}$g$\text{'},\text{'}$h$\text{'},\text{'}$i$\text{'}],[\text{'}$j$\text{'},\text{'}$k$\text{'},\text{'}$l$\text{'}]]$

$>>>$ letter$\_$count$($alphabet$,\text{'}$m$\text{'})$

$0$

$"""$