class ActionView::Template::Error

def source_extract(indentation = 0)
  return unless num = line_number
  num = num.to_i
  source_code = @template.source.split("\n")
  start_on_line = [ num - SOURCE_CODE_RADIUS - 1, 0 ].max
  end_on_line   = [ num + SOURCE_CODE_RADIUS - 1, source_code.length].min
  indent = ' ' * indentation
  line_counter = start_on_line
  return unless source_code = source_code[start_on_line..end_on_line]
  source_code.sum do |line|
    line_counter += 1
    "#{indent}#{line_counter}: #{line}\n"
  end
end