Simple un-obfuscation of some LaTeX internals

Question

Here's an example of some code from LaTeX's \DeclareFontFamily:

\def\reserved@a{#3}
\global
\expandafter\let\csname #1+#2\expandafter\endcsname
       \ifx \reserved@a\@empty
         \@empty
       \else \reserved@a
       \fi

I could be wrong, but to me this looks identical to:

\expandafter\gdef\csname#1+#2\endcsname{#3}

Now, the source for this code comments:

We compare \reserved@a with \@empty If these two are the same we \let the ‘extra’ macro equal to \@empty which is not the same a doing a \let to \reserved@a — the latter would blow one extra memory location rather then reusing the one from \@empty.

But if we're not re-using the contents of \reserved@a after this point (which we're not, as far as I can see), there's no reason to go this roundabout route, right?

Answer 1

For a full answer you'd need to ask one of the people who might have written this (possible Frank Mittelbach or even Leslie Lamport - I wonder if this has come all the way from 2.09). However, what I think is happening is that when \reserved@a is not equal to \@empty then there is no saving, as at some later stage \reserved@a will be needed again. On the other hand, when \reserved@a is equal to \@empty then a memory position is saved, as \@empty never changes and so the two macros always have the same definition as each other. When this code was written, such small savings were probably really important (they all add up).