documentclass{article}
usepackage{graphicx} % for scalebox macro
newcommandrightassn{
  mathrel{ooalign{hss$|$hsscr%
  kern0.9exraise0.2exhbox{scalebox{0.7}{$rangle$}}}}}

newcommandleftassn{mathrel{ooalign{hssraise0.2exhbox{scalebox{0.7}{$langle$}}hsscr%
  kern0.9exhbox{$|$}}}}

begin{document}
$leftassn 1 rightarrow a, 2 rightarrow b rightassn$
end{document}