code: public String getPrefix(PackageElement packageElement) {
      if (packageElement == null) {
        return "";
      }
      String packageName = packageElement.getQualifiedName().toString();
      String prefix = getPrefix(packageName);
      if (prefix != null) {
        return prefix;
      }
  
      prefix = packageLookup.getObjectiveCName(packageName);
      if (prefix == null) {
        prefix = NameTable.camelCaseQualifiedName(packageName);
      }
      addPrefix(packageName, prefix);
      return prefix;
    }
docs: Return the prefix for a specified package. If a prefix was specified
  for the package, then that prefix is returned. Otherwise, a camel-cased
  prefix is created from the package name.