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Assignment to a block inside a matrix

      I have a package that works with the "symplectic unit matrix" 
defined for an n-degree of freedom Hamiltonian system by


I.e. the definition involves the assignment to a 2 by 2 sub-matrix. Can
someone tell me authoritatively whether this programming  construct is
correct or whether I need to rewrite the function  using ReplacePart or
something?   The reason I ask is as follows:

This used to work just fine on both Windows 95 and Unix versions  of
Mathematica.   It still works on Unix (HP version X but there
are problems 
in the Windows95 version (it is according to the  Help/About
Mathematica display and 
$Version evaluates to "Microsoft Windows 3.0 (April 25, 1997)").  

Specifically, I can calculate symJ[1], symJ[2] correctly but  symJ[3]
and symJ[4] cause the kernel crash.  I get a  message 

"This program has performed an illegal opearation and will be shut 
down.  If the problem persists, contact the program vendor."

Clicking on Details gives: 

MATHKERNEL caused an invalid page fault in module MATHDLL.DLL at
0137:0072d9ef. Registers:
EAX=00000002 CS=0137 EIP=0072d9ef EFLGS=00010202 EBX=00a3e6b0 SS=013f
ESP=01a3fad4 EBP=013bd998 ECX=00000001 DS=013f ESI=013661d4 FS=1907
EDX=01309ba0 ES=013f EDI=00000000 GS=0000 Bytes at CS:EIP:
66 8b 47 02 24 07 3c 01 75 35 a1 e4 84 a0 00 8b  Stack dump:
01396ee8 01396848 01365fcc 0131c484 00000006 00000002 00a3e6b0 0043edb4
00000018 0072dae3 013661d4 013bd898 00000002 00000000 013a14dc 0130b8d0

I have checked that this does not happen with Windows version 
which is still installed here.  I have also checked that both versions 
behave the same way on two different PCs with different hardware.

If my function is properly defined then this would appear to be a  bug
introduced in Version  If my function is not properly defined
(and I was just lucky that it worked in the earlier  version) then I
would still expect something better than a kernel crash.

John Jowett

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