$3\mathrm{.}6\mathrm{.}2$: Recursive exploration of all possibilities. Strings are read from input and stored in list cities$\_$to$\_$pick. The recursive function list$\_$all$\_$routes$()$ explores all available routes defined by a source and a destination picked from cities$\_$to$\_$pick. In list$\_$all$\_$routes$(),$ write the base case to output the following items in one line if the size of list picked$\_$cities is $2$:

'From: $\text{'}$

first element in picked$\_$cities

$\text{'},$ To: $\text{'}$

second element in picked$\_$cities.