lazy contiguous subrange indexing

[John W. Eaton]

On 14-Jan-2009, Jaroslav Hajek wrote:

| with a recent tip, I get:
| 
| Elapsed time is 0.697262 seconds.
| Elapsed time is 0.960276 seconds.
| 
| with the new patch, I got:
| 
| Elapsed time is 0.213054 seconds.
| Elapsed time is 0.497459 seconds.
| 
| which means speedups by 228% and 93%.
| P.S. I picked some benefitted functions because measuring directly the
| performance speed-up of the affected indexing expressions is
| meaningless.

Oh, and I should mention that it is great to have the speedup.

Thanks!

jwe