Some labs incompatible with the latest version of J

[ivsukin]

This is not an issue that needs an immediate closing, but some labs are using obsolete primitives like d. and t. Do we need to fix this, so these labs would work properly in the latest version of J? If yes, I could "proofread" labs (especially the ones for beginners) and create a pull request.

[rdm]

I think we'll need to address this, yes.

Also, while the math/calculus addon provides replacements for d. and D. no one has built the library to replace t. and that's another issue which we need to address.