/usr/share/calc/custhelp/devnull is in apcalc-common 2.12.5.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53  | NAME
    devnull - does nothing
SYNOPSIS
    custom("devnull" [, arg ...])
TYPES
    arg		any
    return	null
DESCRIPTION
    This custom function does nothing.	It is intented for testing
    of the general custom interface.
EXAMPLE
    > custom("devnull", foo, bar, baz, 3+4.5i, pi())
LIMITS
    calc must be built with ALLOW_CUSTOM= -DCUSTOM
    calc must be executed with a -C arg.
LIBRARY
    none
SEE ALSO
    custom
## Copyright (C) 1999  Landon Curt Noll
##
## Calc is open software; you can redistribute it and/or modify it under
## the terms of the version 2.1 of the GNU Lesser General Public License
## as published by the Free Software Foundation.
##
## Calc is distributed in the hope that it will be useful, but WITHOUT
## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
## or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU Lesser General
## Public License for more details.
##
## A copy of version 2.1 of the GNU Lesser General Public License is
## distributed with calc under the filename COPYING-LGPL.  You should have
## received a copy with calc; if not, write to Free Software Foundation, Inc.
## 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
##
## @(#) $Revision: 30.1 $
## @(#) $Id: devnull,v 30.1 2007/03/16 11:10:04 chongo Exp $
## @(#) $Source: /usr/local/src/bin/calc/custom/RCS/devnull,v $
##
## Under source code control:	1997/03/09 17:49:12
## File existed as early as:	1997
##
## chongo <was here> /\oo/\	http://www.isthe.com/chongo/
## Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/
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