THe length of a Source(File) is always measured from its start. In this sense,
the length is like a position on the timeline, which is a duration with an
implicit origin, or a Region start, also a duration with an implicit origin (in
that case the start of the Source). There is no good reason for using
a timecnt_t for this value, because the position component of a timecnt_t
(the origin for the duration) is implicit and always zero. So we make
this property into a timepos_t, and include a number of asserts() to check
for common possible coding errors related to the time domain
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