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  Sub-Indexes:

  Sub-indexes are indexes that are built based upon another index, usually
  a conditional index.  Instead of plowing through the entire database, a
  sub-index includes only the records that are contained in the index upon
  which it is based.  This adds up to substantial time savings when all you
  want is to re-arrange the a conditional index.

  Sub-indexes are created with the INDEX command using the SUBINDEX keyword
  at the end of an INDEX ON command, or by just using the SUBINDEX command.
  This tells the SIx Driver to use the current index to build the
  new one.

  Note that sub-indexes do NOT inherit the condition from the original index.

  NOTE: Sub-Indexes are considered non-updatable and are normally intended
        to be used as temporary indexes.  What this means is, if you change
        keys values in your database after creating or opening a SUBINDEX,
        the keys in the SUBINDEX will not change.  If you want to force a
        SUBINDEX to be updatable, you can do so by also making it a condi-
        tional index by using a FOR clause.