s = "123456"
r1 = "abc"
s.loverwrite(r1) # => "abc456"
s.coverwrite(r1) # => "12abc6"  (or "1abc56")
s.roverwrite(r1) # => "123abc"

r2 = "abcdefghi"
s.loverwrite(r2) # => "abcdef"
s.coverwrite(r2) # => "bcdefg"  (or "cdefgh")
s.roverwrite(r2) # => "defghi"

class String
  def loverwrite(s)
    dup.tap{|result| result[0, s.length] = s[0, length]}
  end
end