  Three-point vertex diagram plays a key role in the whole renormalization
program of several QFT (quantum field theory) models such as QED, QCD, the
Standard Model of eletroweak interactions and so forth. The exact analytic
result for the triangle diagram therefore is fundamental.
  In this work we calculate in two different ways a two-point two-loop massless
Feynman diagram using what we call a "gluing" technique in the context of NDIM
(Negative Dimensional Integration Method). The two-loop diagram in question can
be "glued" in two different ways and we show that both yield the same result
and reproduce the one calculated via NDIM for the complete diagram, which, of
course, is equivalent to the exact solution obtained by normal positive
dimensional calculation.
  Furthermore, in the process we conclude that the usual massless off-shell
triangle diagram result does not hold anymore and present a new solution for it
with only three hypergeometric functions $F_{4}$.
