Add perm * sparse, perm \ sparse, sparse * perm, and sparse / perm operations. Nothing terribly fancy in any of this. There probably is some mechanism for using the permutation vectors and some assign or index method in the sparse classes, but I've never understood all the intricacies. I'm opting for a simple implementation at the cost of possibly duplicating some functionality.

#!/bin/sh
#
# Like mv $1 $2, but if the files are the same, just delete $1.
# Status is 0 if $2 is changed, 1 otherwise.

if test -r $2; then
  if cmp $1 $2 > /dev/null; then
    echo $2 is unchanged
    rm -f $1
  else
    mv -f $1 $2
  fi
else
  mv -f $1 $2
fi