the ti57 appeared in 1977 and was the least sophisticated of the 57,58 &
range but more competent than the earlier ti-51-iii
(ti-55). it is programmable with a 50 step volatile memory.
the program steps are fully merged keystrokes, which is good because
several common operations require three key presses (eg INV 2nd x==t).
it has all the expected program features, subroutines, labels, conditional
branches and decrement loops (dsz).
50 steps wasnt a lot, but the versatility of conditionals and especially
the `t' register meant that useful programs could be written.
moreover, the machine was affordable in comparison to the much more
expensive ti-58 and ti-59