def get_file_context(filename, lineno, context)
      return nil, nil, nil unless valid_path?(filename)
      lines = Array.new(2 * context + 1) do |i|
        getline(filename, lineno - context + i)
      end
      [lines[0..(context - 1)], lines[context], lines[(context + 1)..-1]]
    end
def get_file_context(filename, lineno, context)
      return nil, nil, nil unless valid_path?(filename)
      lines = Array.new(2 * context + 1) do |i|
        getline(filename, lineno - context + i)
      end
      [lines[0..(context - 1)], lines[context], lines[(context + 1)..-1]]
    end
Any linecache you provide to Raven must implement this method.
 Returns an Array of Strings representing the lines in the source
 file. The number of lines retrieved is (2 * context) + 1, the middle
 line should be the line requested by lineno. See specs for more information.