96c31b2618
implement (better) falback code inside srandomdev() itself. Change return type from int to void (binary compatibility surprisely achieved). Userland code will be changed soon.
190 lines
5.9 KiB
Groff
190 lines
5.9 KiB
Groff
.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)random.3 8.1 (Berkeley) 6/4/93
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.\"
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.Dd June 4, 1993
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.Dt RANDOM 3
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.Os BSD 4.2
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.Sh NAME
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.Nm random ,
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.Nm srandom ,
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.Nm srandomdev ,
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.Nm initstate ,
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.Nm setstate
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.Nd better random number generator; routines for changing generators
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.Sh SYNOPSIS
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.Fd #include <stdlib.h>
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.Ft long
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.Fn random void
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.Ft void
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.Fn srandom "unsigned long seed"
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.Ft void
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.Fn srandomdev void
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.Ft char *
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.Fn initstate "unsigned long seed" "char *state" "long n"
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.Ft char *
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.Fn setstate "char *state"
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.Sh DESCRIPTION
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The
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.Fn random
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function
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uses a non-linear additive feedback random number generator employing a
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default table of size 31 long integers to return successive pseudo-random
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numbers in the range from 0 to
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.if t 2\u\s731\s10\d\(mi1.
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.if n (2**31)\(mi1.
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The period of this random number generator is very large, approximately
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.if t 16\(mu(2\u\s731\s10\d\(mi1).
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.if n 16*((2**31)\(mi1).
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.Pp
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The
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.Fn random
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and
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.Fn srandom
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functions have (almost) the same calling sequence and initialization properties as the
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.Xr rand 3
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and
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.Xr srand 3
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functions.
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The difference is that
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.Xr rand 3
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produces a much less random sequence \(em in fact, the low dozen bits
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generated by rand go through a cyclic pattern. All the bits generated by
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.Fn random
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are usable. For example,
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.Sq Li random()&01
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will produce a random binary
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value.
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.Pp
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Like
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.Xr rand 3 ,
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.Fn random
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will by default produce a sequence of numbers that can be duplicated
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by calling
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.Fn srandom
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with
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.Ql 1
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as the seed.
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.Pp
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The
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.Fn srandomdev
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routine initialize a state array using
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.Xr urandom 4
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random number device which returns good random numbers,
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suitable for cryptographic use.
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Note that this particular seeding
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procedure can generate states which are impossible to reproduce by
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calling
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.Fn srandom
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with any value, since the succeeding terms in the
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state buffer are no longer derived from the LC algorithm applied to
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a fixed seed.
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.Pp
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The
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.Fn initstate
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routine allows a state array, passed in as an argument, to be initialized
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for future use. The size of the state array (in bytes) is used by
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.Fn initstate
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to decide how sophisticated a random number generator it should use \(em the
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more state, the better the random numbers will be.
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(Current "optimal" values for the amount of state information are
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8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
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the nearest known amount. Using less than 8 bytes will cause an error.)
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The seed for the initialization (which specifies a starting point for
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the random number sequence, and provides for restarting at the same
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point) is also an argument.
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The
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.Fn initstate
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function
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returns a pointer to the previous state information array.
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.Pp
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Once a state has been initialized, the
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.Fn setstate
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routine provides for rapid switching between states.
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The
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.Fn setstate
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function
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returns a pointer to the previous state array; its
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argument state array is used for further random number generation
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until the next call to
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.Fn initstate
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or
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.Fn setstate .
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.Pp
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Once a state array has been initialized, it may be restarted at a
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different point either by calling
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.Fn initstate
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(with the desired seed, the state array, and its size) or by calling
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both
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.Fn setstate
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(with the state array) and
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.Fn srandom
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(with the desired seed).
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The advantage of calling both
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.Fn setstate
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and
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.Fn srandom
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is that the size of the state array does not have to be remembered after
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it is initialized.
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.Pp
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With 256 bytes of state information, the period of the random number
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generator is greater than
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.if t 2\u\s769\s10\d,
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.if n 2**69
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which should be sufficient for most purposes.
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.Sh AUTHOR
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Earl T. Cohen
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.Sh DIAGNOSTICS
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If
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.Fn initstate
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is called with less than 8 bytes of state information, or if
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.Fn setstate
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detects that the state information has been garbled, error
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messages are printed on the standard error output.
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.Sh SEE ALSO
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.Xr rand 3 ,
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.Xr srand 3 ,
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.Xr urandom 4
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.Sh HISTORY
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These
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functions appeared in
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.Bx 4.2 .
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.Sh BUGS
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.Pp
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About 2/3 the speed of
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.Xr rand 3 .
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.Pp
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The historical implementation used to have a very weak seeding; the
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random sequence did not vary much with the seed.
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The current implementation employs a better pseudo-random number
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generator for the initial state calculation.
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