  The pinched/non-pinched classification of intersections of causal
singularities of propagators in Minkowski space is reconsidered in the context
of the theory of asymptotic operation as a first step towards extension of the
latter to non-Euclidean asymptotic regimes. A highly visual
distribution-theoretic technique of singular wave fronts is tailored to the
needs of the theory of Feynman diagrams. Besides a simple derivation of the
usual Landau equations in the case of the conventional singularities, the
technique naturally extends to other types of singularities e.g. due to linear
denominators in non-covariant gauges etc. As another application, the results
of Euclidean asymptotic operation are extended to a class of quasi-Euclidean
asymptotic regimes in Minkowski space.
