HP -25 Owner's Handbook Manual page 27

It is this unique system that gives you all these calculating ad-
vantages whether you're writing keystrokes for an HP-25 pro-
gram or using the HP-25 under manual control:
You never have to work with more than one function at a
time. The HP-25 cuts problems down to size instead of
making them more complex.
Pressing a function key immediately executes the function.
You work naturally through complicated problems, with
fewer keystrokes and less time spent.
Intermediate results appearas they are calculated. There
are no ''hidden'' calculations, and you can check each step
as you go.
Intermediate results are automatically handled. You don't
have to write down long intermediate answers when you
work a problem.
Intermediate answers are automatically inserted into the
problem on a last-in, first-out basis. You don't have to
remember where they are and then summon them.
You can calculate in the same order you do with pencil and
paper. You don't have to think the problem through ahead
of time.
The HP system takes a few minutes to learn. But you'll be
amply rewarded by the ease with which the HP-25 solves the
longest, most complex equations. With HP, the investment of a
few moments of learning yields a lifetime of mathematical bliss.