add vercos(), avercos(), covercos(), acovercos()

Improved trig help files.

Added new vercos(x, [,eps]) for versed cosine and covercos(x, [,eps])
for inverse versed cosine.

Added new avercos(x, [,eps]) for inverse versed cosine and acovercos(x, [,eps])
for inverse coversed cosine.

help/avercos

@@ -0,0 +1,63 @@
+NAME
+    avercos - inverse versed trigonometric cosine
+
+SYNOPSIS
+    avercos(x [,eps])
+
+TYPES
+    x		number (real or complex)
+    eps		0 < real < 1, defaults to epsilon()
+
+    return	real
+
+DESCRIPTION
+    Returns the inverse versed trigonometric cosine of x to a multiple of eps with error less in
+    absolute value than .75 * eps.
+
+    This function is sometimes called averc, or arcvercos, is equivalent to:
+
+	avercos(x) = acos(x - 1)
+
+EXAMPLE
+    ; print avercos(.5, 1e-5), avercos(.5, 1e-10), avercos(.5, 1e-15), avercos(.5, 1e-20)
+    2.0944 2.0943951024 2.094395102393195 2.09439510239319549231
+
+    ; print avercos(2), avercos(-5), avercos(2 + 3i)
+    0 3.14159265358979323846-2.47788873028847500481i 1.26319267726418536931-1.86416154415788242834i
+
+LIMITS
+    0 < eps < 1
+
+LINK LIBRARY
+    NUMBER *qavercos_or_NULL(NUMBER *q, NUMBER *epsilon);
+    NUMBER *qavercos(NUMBER *q, NUMBER *epsilon);
+    COMPLEX *c_avercos(COMPLEX *c, NUMBER *epsilon);
+
+SEE ALSO
+    sin, cos, tan, cot, sec, csc
+    asin, acos, atan, acot, asec, acsc
+    versin, coversin, vercos, avercos
+    aversin, acoversin, covercos, acovercos
+    epsilon
+
+## Copyright (C) 2023  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.
+##
+## Under source code control:	2023/09/05 23:40:24
+## File existed as early as:	2023
+##
+## chongo <was here> /\oo/\	http://www.isthe.com/chongo/
+## Share and enjoy!  :-)	http://www.isthe.com/chongo/tech/comp/calc/