4. w We say that a symbol of a grammar is useless if it does not appear in the derivation of a strings of terminals. Formally, A is useful if there exists a derivation S =>* x A y =>* w where w is a string of terminals. A is useless if it is not useful. Note that a useful symbol can be either a terminal or a non- terminal. Also, a useless symbol can be a terminal or a non-terminal. Consider the grammar SAb CIDCA A a C E DIE CCA DDIE Which are the useful symbols of this grammar? For every useful symbol, give a derivation that shows that the symbol is useful