Mrthfx

I'm reading source2e.pdf and I have problems in understanding the sense of the definition of LaTeX 2ε's esphack:

Commands like label{...} , which themselves do not/shall not produce any visible output, might in the source/in the .tex-input-file be surrounded by spaces. If so, you don't want two space-tokens as this would double the horizontal glue.

If I got it right, @bsphack and @esphack are there for avoiding the coming into being of a space-token after tokenizing and processing the command in case there already came one into being before tokenizing and processing the command.

So let's look at the definition of @bsphack:

> @bsphack=macro:

->relax ifhmode @savsk lastskip @savsf spacefactor fi .

l.1    show@bsphack

In words:

When in horizontal mode or in restricted horizontal mode, then save the value of lastskip to @savsk and save the value of spacefactor to @savsf.

Now let's look at the definition of @esphack:

> @esphack=macro:

->relax ifhmode spacefactor @savsf ifdim @savsk >z@ ifdim lastskip =z

@ nobreak hskip z@skip fi ignorespaces fi fi .

l.2    show@esphack

In words:

When in horizontal mode or in restricted horizontal mode, then:

• restore the value of spacefactor to the value saved as savsf.


• In case @savsk is larger than zero, i.e., in case there was some horizontal glue before carrying out the command which at its start called @bsphack, then do
  ifdim lastskip =z@ nobreak hskip z@skip fi ignorespaces
  .

I understand that ignorespaces will make sense as there was already some horizontal glue before carrying out the command which at its start called @bsphack.

But I don't understand what the
ifdim lastskip =z@ nobreak hskip z@skip fi
-part is good for.

What is the sense of this?

In the situation where this is carried out, the saved value in @savsk is larger than z@, thus one can conclude that something from within the command which at its start called @bsphack did change the value of lastskip to z@.

But doing something like nobreakhskipz@skip will not revert that change/will not restore lastskip to its previous value.

Thus: What is the gist/sense/benefit of performing that hskip of zero-width?

If you wish lastskip to be restored, shouldn't it then be something like:

def@esphack{%

  relax

  ifhmode

    spacefactor@savsf

    ifdim@savsk>z@

      ifdimlastskip=z@

        nobreak

        hskip-@savsk

        nobreak

        hskip@savsk

        % the total skip is zero and lastskip is restored.

      fi

      ignorespaces

    fi

  fi

}%

Or perhaps just

def@esphack{%

  relax

  ifhmode

    spacefactor@savsf %now spacefactor is restored.

    nobreak

    hskip-@savsk

    nobreak

    hskip@savsk

    % the total skip is zero and lastskip is restored.

    ifdim@savsk>z@

      ignorespaces

    fi

  fi

}%

Is it possible that in some previous release, it was done this way and in later releases somebody erroneously "optimized" the
nobreakhskip-@savsknobreakhskip@savsk to nobreakhskipz@skip, overlooking that this won't restore lastskip any more?

If I understand correctly, you need to restore lastskip correctly as otherwise things with consecutive sequences of @bsphck..@esphack, e.g., label{foo}label{bar} won't work correctly.