def at(offset)
      if loaded? || partially_loaded?(offset)
        super
      elsif offset == 0
        first
      elsif offset > 0
        first(:offset => offset)
      elsif offset == -1
        last
      else
        last(:offset => offset.abs - 1)
      end
    end
def at(offset)
      if loaded? || partially_loaded?(offset)
        super
      elsif offset == 0
        first
      elsif offset > 0
        first(:offset => offset)
      elsif offset == -1
        last
      else
        last(:offset => offset.abs - 1)
      end
    end
Lookup a Resource from the Collection by offset

 @param [Integer] offset
   offset of the Resource in the Collection

 @return [Resource]
   Resource which matches the supplied offset
 @return [nil]
   No Resource matches the supplied offset

 @api public