Here is the main code which is concerned with initialisations and output of messages explaining how far the matrix construction has progressed.
( @count@
@@=@ @ @@@@@@@@@ @MATRIX@ @@
@@@@@@rotation @@@@@ =@i @
@@i#1#@#1@ @ii @\xymatrix: <rows> expected
@@ii#1@pretypeset#1@measureit @setup @typeset#1@insert @@@ )
: Proper nesting that ensures that the matrix state is reestablished after the current one has been typeset.
Each subprocedure is explained below.