On Arrows (over/under - end

\renewcommand{\xrightarrow}[2][]{%
    \if :#1:
       \HCode{<.msup limits base .>}%
         \rightarrow
         <.msup limits script super .>#2%
         <.close limits sup .>%
    \else
       \HCode{<.start limits msubsup base .>}%
         \leftarrow
         <.msubsup limits sub .>#1%
         <.msubsup limits sup .>#2%
         <.close limits script .>%
    \fi
}
\renewcommand{\xleftarrow}[2][]{%
    \if :#1:
       \HCode{<.msup limits base .>}%
         \leftarrow
         <.msup limits script super .>#2%
         <.close limits sup .>%
    \else
       \HCode{<.start limits msubsup base .>}%
         \leftarrow
         <.msubsup limits sub .>#1%
         <.msubsup limits sup .>#2%
         <.close limits script .>%
    \fi
}

-_-_-