deletions | additions       diff --git a/Concepts.tex b/Concepts.tex  index e6f21cd..ed732e1 100644  --- a/Concepts.tex  +++ b/Concepts.tex 
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as excitation spots.  Each array contains timestamps from both donor (D) and acceptor (A) channels.  When alternating excitation lasers are used (ALEX measurements)~\cite{Lee_2005}, a further distinction between photons emitted during the D or A excitation periods can be made. In FRETBursts, the corresponding sets of photons are called ``photon streams'' and are specified with a \verb|Ph_sel| object  (\href{http://fretbursts.readthedocs.org/en/latest/ph_sel.html}{link}).  In non-ALEX smFRET data, there are 3 photon streams