StringBeginsWith( const nsACString& aSource, const nsACString& aSubstring, const nsCStringComparator& aComparator )
StringBeginsWith( const nsAString& aSource, const nsAString& aSubstring, const nsStringComparator& aComparator )
StringBeginsWith( const nsACString& aSource, const nsACString& aSubstring,                  const nsCStringComparator& aComparator )  {    nsACString::size_type src_len = aSource.Length(),                          sub_len = aSubstring.Length();    if (sub_len > src_len)      return PR_FALSE;    return Substring(aSource, 0, sub_len).Equals(aSubstring, aComparator);  }