  We illustrate a duality relation between one-loop integrals and single-cut
phase-space integrals. The duality relation is realised by a modification of
the customary +i0 prescription of the Feynman propagators. The new prescription
regularizing the propagators, which we write in a Lorentz covariant form,
compensates for the absence of multiple-cut contributions that appear in the
Feynman Tree Theorem. The duality relation can be extended to generic one-loop
quantities, such as Green's functions, in any relativistic, local and unitary
field theories.
