TI 58/59 ROM modules and OP instructions

[Dieter]

I have been looking at some programs in the TI58/59 ML-01 master library module, and there is a detail that makes makes me wonder: There are numerous places where the OP 2x and OP 3x instructions that increment / decrement a data register would have made sense, but instead a more elaborate command sequence has been used, like x<>t 1 INV SUM 01 x<>t instead of a simple OP 31. In a similar way the sign function OP 10 is avoided, instead this is accomplished by dividing x by its absolute value (which also requires special care for x=0).

So it looks like the OP commands are deliberately avoided in TI's ROM modules. But why? Is there a certain technical reason for this?

Dieter