sci.physics.particle

Hello,

Can anyone explain to me that why is ?? added to the euclidean propagator in
k or p space?
Without it, the Euclidean porpagator has poles on real axis and it can be
integrated
with countour integration as I see it by deforming the countour. Then why
are the poles shifted from real axis
by adding ?? ?? OR is it postulated for reasons not obvious until the one
goes from Euclidean to Minkowski
and t in introduced?

Is ?? always added or it may also be subtracted?

The course I am following is Introduction to QFT.

Thanks.