The software I am writing allows a user to combine multiple 
potentially-overlapping audio clips by specifying an in and out point for each 
clip, plus when in the final stream that clip starts. I'm trying to string 
together a single filter graph that will do this and am struggling with the 
start point bit. My thought was to take each of the audio files and construct a 
filter chain for them that inserts silence of the required length at the 
beginning, then the atrim'ed audio clip, then an apad to the end of the 
required length, so (for just a single audio file), something like:

(some kind of silence-generator here)[delay0];[in0]atrim=start=1:end=3,apad 
[end0];[delay0][end0]concat=n=2,v=0,a=1[final0]

I can't seem to figure out how to make the concat work, though. No matter what 
technique I use to generate the silence (or non-silence, I've tried using sine 
and anoisesrc), I am getting an "invalid argument". I've tried playing around 
with the ffmpeg exe on the command line, but still can't make heads or tails of 
concat. What am I missing? Or is there a better way to do what I am describing?
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