Q1 (a) (8 marks)

A double-lettered word contains two (or more) consecutive identical letters. For example, 'abba' and 'pizza' are double-lettered words but 'and' and 'every' are not.

Complete the following problem definition for a function *double lettered* that accepts a single word consisting only of lowercase letters as input, and returns true if the word is double lettered, or false if it is not. For the purposes of this question we do not care whether the word exists in a dictionary, just that it is a string of lowercase letters.