lib/fakefs/pathname.rb
def +(other)
      other = Pathname.new(other) unless other.is_a?(Pathname)
      Pathname.new(plus(@path, other.to_s))
    end
def +(other)
      other = Pathname.new(other) unless other.is_a?(Pathname)
      Pathname.new(plus(@path, other.to_s))
    end
Pathname#+ appends a pathname fragment to this one to produce a new
 Pathname
 object.

   p1 = Pathname.new("/usr")      # Pathname:/usr
   p2 = p1 + "bin/ruby"           # Pathname:/usr/bin/ruby
   p3 = p1 + "/etc/passwd"        # Pathname:/etc/passwd

 This method doesn't access the file system; it is pure string
 manipulation.