// V[selpos] contains the position id you are looking for. e.g. "  20"
// Add 0 to delete leading space
Set V[selpos] &V[selpos] + 0

Set V[index] 1
label next_line

Set V[pos] "&cell[All items,Pos,&V[index]]" 

// end of displayed lines?
if not V[pos]
  Set V[index] 0
  goto end_of_table
endif

// Add 0 to delete leading space
Set V[pos] &V[pos] + 0

// Position matching?
if V[pos=&V[selpos]]
  goto end_of_table
endif

// Increase index 
Set V[index] &V[index] + 1

goto next_line


label end_of_table

// Mark line (if found)
if V[index>0]
  Set cell[All items,0,&V[index]] "X"
endif