Given the following dafny datatype:

datatype MapSet $=$ MapSet $($s : map$)$

and given the two functions:

function insert $($m:MapSet, x:T$)$: MapSet

$\{$

$//$ Replace with your definition

MapSet $($m$.$s$[$x :$=$ true$])$

$\}$

function delete $($m:MapSet, x:T$)$: MapSet

$\{$

$//$ Replace with your definition

MapSet $($m$.$s$[$x :$=$ false$])$

$\}$

Please code the following function:

function size $($m:MapSet$)$: int $\{$

$//$ return the size of the map counting only the elements $($keys$)$ with values "true"

$\}$

This problem is supposedly solvable using the maps function provided in the image below

predicate Empty$3$:$=20$:$=k$:Umm