[smaddox]

I can usually get away with visualizing indices as being just before a memory cell (i.e. on the boundary between cells), whereas lengths are spans of cells. For example:

          | H | e | l | l | o |

    Index 0   1   2   3   4   5

   Length |<--------5-------->|

This makes it easy to visualize that 1-based indexing has indices centered on the memory cell, so calculating a span requires reducing the subtractor by 1.