Given a string s, write expressions for:

• The string consisting of the first letter

• The string consisting of the first and last letter

• The string consisting of all but the first and last letter

• The first half of the string (discarding the middle letter if the string has odd length)

• The second half of the string (discarding the middle letter if the string has odd length)

• The string consisting of the middle letter if the string has odd length, or the middle two letters otherwise