end
function starts_with(s, prefix)
  if #s < #prefix then
    return false
  end
  for i=1,#prefix do
    if s:sub(i,i) ~= prefix:sub(i,i) then
      return false
    end
  end
  return true
end
function ends_with(s, suffix)
  if #s < #suffix then
    return false
  end
  for i=0,#suffix-1 do
    if s:sub(#s-i,#s-i) ~= suffix:sub(#suffix-i,#suffix-i) then
      return false
    end
  end
  return true

-- TODO: avoid the expensive reverse() operations
-- Particularly if we only care about literal matches, we don't need all of string.find

    if edge.prefix and find(frag, edge.prefix, nil, --[[plain]] true) == 1 then

    if edge.prefix and starts_with(frag, edge.prefix) then

    if edge.suffix and rfind(frag, edge.suffix, nil, --[[plain]] true) == #frag - #edge.suffix + 1 then

    if edge.suffix and ends_with(frag, edge.suffix) then

end
function trim(s)
  return s:gsub('^%s+', ''):gsub('%s+$', '')
end
function ltrim(s)
  return s:gsub('^%s+', '')
end
function rtrim(s)
  return s:gsub('%s+$', '')