QUESTION $2$

Here is a definition for a datatype called Tree which can be used to represent expression trees.

datatype Tree $=$ Leaf of int $|$ Add of Tree $*$ Tree $|$ Sub of Tree $*$ Tree ;

Which of the following represents a complete implementation to evaluate any. valid expression tree that takes a Tree and retur integer?

fun eval $($Leaf$($c$))=c$

$|$ Leaf $($c$))$ Leaf $(c)$;

fun eval $($Leaf $($c$))=$ c

$|$ eval$(Leaf(c,d))=$eval$(Add(c),$Sub$(d))$;

fun eval $($Leaf $($c$))=c$

$($ eval$(Add(c,d))=$eval$(c)+$eval$(d)$

$|$ eval$(Sub(c,d))=$ eval $($c$)-$eval$(d)$;

fun eval $($Leaf $($c$))=c$

I eval$(c,d)=c+d$

I eval$(c,d)=c-d$;

fun eval $(c)=c$

$|eval(c)|=$eval$(Add(3,4))$

$|$ eval$(c)=$eval$(Sub(3,4))$;