A Crossed Beam Scattering Study of Reactions in the System Acetylene Cation-acetylene: Formation of C.sub.2./sub.HD.sup.+ú./sup. in C.sub.2./sub.D.sub.2./sub..sup.+ú./sup. + C.sub.2./sub.H.sub.2./sub. and Formation of C.sub.4./sub.H.sub.3./sub..sup.+./sup. and C.sub.4./sub.H.sub.2./sub..sup.+ú./sup. in C.sub.2./sub.H.sub.2./sub..sup.+ú./sup. + C.sub.2./sub.H.sub.2./sub. Collisions