Consider grammar G_4 = {V. sigma R, (EXPR)). V is {(EXPR), (TERM), (FACTOR)} and sigma is {a +, x (, )}. The rules are (EXPR) rightarrow (EXPR)+(TERM) | (TERM) (TERM) rightarrow (TERM) times (FACTOR) | (FACTOR) (FACTOR) rightarrow ((EXPR)) | a The two strings a + axa and (a + a)xa can be generated with grammar G_4. The parse trees are shown in the following figure