Glad to oblige. :-)
If one of the inputs to SENTENCE is the empty list, it just returns
the other input, so the *extra* calls cost only the time for a call
to a primitive procedure, which is nonzero but fairly small. The
calls that really do work are more expensive, though, because of the
extra call to LIST and a little recopying inside SENTENCE.
I expect that all of these costs are swamped by the cost of parsing
and evaluating the program text! In interpreted versions of Logo
(all but Object Logo, I think) this happens for each invocation.
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