PUNICODE_PREFIX_TABLE_ENTRY
RtlFindUnicodePrefix(
  IN  PUNICODE_PREFIX_TABLE PrefixTable,
  IN  PUNICODE_STRING FullName,
  IN  ULONG CaseInsensitiveIndex
  );

Routine Description:

    This routine finds if a full name has a prefix in a prefix table.
    It returns a pointer to the largest proper prefix found if one exists.

Arguments:

    PrefixTable - Supplies the prefix table to search

    FullString - Supplies the name to search for

    CaseInsensitiveIndex - Indicates the wchar index at which to do a case
        insensitive search.  All characters before the index are searched
        case sensitive and all characters at and after the index are searched
        insensitive.

Return Value:

    PPREFIX_TABLE_ENTRY - a pointer to the longest prefix found if one
        exists, and NULL otherwise