Communications of the ACM

nator markers, which he dismissed as unnecessary, is still a good idea. The language used in commercial flight software requires terminating if statements with end if;, records with end record;, case statements withend case;, and so on. Many constructs can be labeled with a name that then appears in the terminator— end loop Name;. This is because there is evidence block terminators are a common source of error, and such labeling reduces such errors, even for short blocks. Short terminators (such as a brace) are a greater source of error than long terminators (such as end), which is why braces are avoided. Safety and readability are designed into the language rather than as an afterthought.

The economic pressures to expand liability to more software would thus be expected to lead to a reduction in the use of languages with braces, along with an associated increase in readability.

Jeffrey R. Carter, Mesa, AZ

A Log Graph Too Far?

I really enjoyed George V. Neville-
Neil’s article “Time Is an Illusion
Lunchtime Doubly So.” (Jan. 2016)
and am a big fan of his Kode Vicious
columns as well. However, I could
not fathom the intriguing figure (see
it here) in the article, labeled simply
“PTP log graph,” nor could I find any
reference to it in the text that might
shed light on what property of the Pre-
cision Time Protocol it is supposed to
illustrate. Not being an electrical en-
gineer myself, I thought it might be
something obvious to someone in the
field, yet a friend with a Ph.D. in elec-
trical engineering was equally flum-
moxed. It was such an interesting
chart I am keen to understand what
it means—if I can. Prof. Neville-Neil,
can you enlighten me?
John Beresniewicz, Half Moon Bay, CA

Author Responds:

The PTP Log Graph in the figure showed the offset of a system clock that is not regulated by an outside time source (such as NTP and PTP). Without an outside time source, the clock wanders away from where we would expect it to be if the system’s crystal oscillator was more stable, which it is not.

George V. Neville-Neil, Brooklyn, NY

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