after using SetMandelstam[..], the Mandelstam variables are then plugged in
on every possible occasion.
However, I only want to use them at a particular point in my Mathematica program,
and after this, I want to switch back to unevaluated momenta.
So, how do I prohibit the automatic replacement of the momentum products
by Mandelstam-expressions after using SetMandelstam for one time?
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