class Kramdown::Parser::RFC2629Kramdown

def self.idref_cleanup(href)
  # can't start an IDREF with a number or reserved start
  if href =~ / /
    warn "** space(s) in cross-reference '#{href}' -- are you trying to use section references?"
  end
  href.gsub(/\A(?:[0-9]|section-|u-|figure-|table-|iref-)/) { "_#{$&}" }
end