Give a possible regular expression that generates the language L consisting of all words in which all words - either contain no as at all, or - when there are as, they occur in clumps of three or more as, and - when there are as, there are an even number of such clumps of as. For instance, words such as b, bbbbbaaaabaaaaabbb and aaabbbaaaaaabaaabaaab are words in this language, and words such as aaaa and aaabaa are not words in the language.