$2\mathrm{.}13$ Consider the following grammar:

stmt $$ assignment

$$ subr$\_$call

assignment $$ id :$=$ expr

subr$\_$call $$ id $($ arg$\_$list$)$

expr $$ primary expr$\_$tail

expr$\_$tail $$ op expr

$$

primary $$ id

$$ subr$\_$call

$($ expr $)$

$op+|-|**\frac{|}{?}$

arg$\_$list $$ expr args$\_$tail

args$\_$tail $,$ arg$\_$list

$$

$($a$)$ Construct a parse tree for the input string foo $(a,b).$

$($b$)$ Give a canonical $($right$-$most$)$ derivation of this same string.