fill in the $?$ marks

$"""$

fij $=$ firstdiff$\_$ij$($i$,$ j$,$ n$)$

Inputs:

$-`$i$`,$ row index

$-`$j$`,$ column index

$-`$n$`,$ length of each side of desired first difference matrix

Output: $`$fij$`$ is the $(`$i$`,`$j$`)-$th element of the $`$n$`-$dimensional first difference matrix.

$"""$

function firstdiff$\_$ij$($i::Integer, j::Integer, n::Integer$)$

if $??$

$??$

elseif $??$

$??$

elseif $(??)$ && $(??)$

$??$

else

$??$

end

return fij

end