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 → + | - | * | /
arg list → expr arg_ 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.

(c) Prove that the grammar is not LL(1).

(d) Modify the grammar so that it is LL(1).