OutputProxy is used by some UGens as a place holder for multiple outputs. There is no reason for a user to create an OutputProxy directly.
var out;
// Pan2 uses an OutputProxy for each of its two outputs.
out = Pan2.ar(WhiteNoise.ar, 0.0);
out.postln;
The UGen that is the source for this OutputProxy.
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var left, right;
// Pan2 uses an OutputProxy for each of its two outputs.
# left, right = Pan2.ar(WhiteNoise.ar, 0.0);
left.source.postln;
The source
method is also defined in Array, so that the source can be obtained this way as well:
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var out;
// Pan2 uses an OutputProxy for each of its two outputs.
out = Pan2.ar(WhiteNoise.ar, 0.0);
out.postln;
out.source.postln;