    EXTRACT:  Extract items at random from a data file.

    Extract is called by the command:

      extract frac full-file extract-file remainder-file [ seed ]

    It reads a file of items (one per line) from 'full-file', chooses a
    fraction 'fract' of items for extraction, writes the chosen items to
    "extract-file", and writes the items not chosen to remainder-file.
    A seed for the random number generator may optionally be specified
    (the default is 1).

    The total number of items read, the fraction extracted, and the number
    extracted are reported on standard error.  The number actually
    extracted will not be the exact fraction if this is not an integer.

    Calls of "extract" may be cascaded to produce useful effects.

                Copyright (c) 1995-2003 by Radford M. Neal