  Based on the generating functional method with an external source function, a
useful constraint on the source function is proposed for analyzing the one- and
two-loop world-line Green functions. The constraint plays the same role as the
momentum conservation law of a certain nontrivial form, and transforms
ambiguous Green functions into the uniquely defined Green functions. We also
argue reparametrizations of the Green functions defined on differently
parameterized world-line diagrams.
