shrink(i,j) is
@$\bb$@ let h :- i@$<$@h@$<$@j 
   @$\text{\bf in}$@ if
        x @$<$@ S.h @$\rightarrow$@ j:=h
       @$\assync\ S.h\le x$@ @$\rightarrow$@ i:=h
      fi
   tel
@$\be$@
