function formatSubPath (path) {
  var trimmed = path.trim();
  // invalid leading 0
  if (path.charAt(0) === '0' && isNaN(path)) { return false }

  return isLiteral(trimmed) ? stripQuotes(trimmed) : '*' + trimmed
}
Format a subPath, return its plain form if it is
a literal string or number. Otherwise prepend the
dynamic indicator (*).