I am having trouble with this question in my Programming Languages course.

Consider the following grammar:

stmt assignment

assignment id := expr

subr_call id ( arg_list )

expr primary expr tail

expr_tail op expr

primary id

subr call

subr_call

( expr )

op + | - | * | /

arg_list expr args_tail

args_tail , arg_list

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