  We present an efficient algorithm for constructing all the matrix elements of
covariant tensor currents of massless particles of arbitrary spins in the
covariant formulation. The construction of matrix elements can be taken simply
by assembling the basic matrix elements which are derived from the basic
three-point vertices. We obtain the selection rules for the decay of an
off-shell massive particle into two identical massless particles which are the
generalization of the Landau-Yang (LY) theorem. After showing how to identify
fully the discontinuity between the matrix elements for the spacelike and
lightlike momentum transfers, we derive all the matrix elements of conserved
tensor currents of massless particles including high spins, from which the
Weinberg and Witten (WW) theorem is automatically extracted with additional
limits on the particles.
