Computer Theory: Context-Free Languages

A substitution is the action of taking a language L and two strings of terminals called sa and sb and changing every word of L by substituting the string sa for each a and the string sb for each b in the word. This turns L into a completely new language. Let us say, for example, that L was the language defined by the regular expression a (bab*+aa)* and say that Then L would become the language defined by the regular expression (i) Prove that after any substitution any regular language is still regular. (ii) Prove that after any substitution a CFL is still context-free