Arrows are provided to supplement \xleftarrow and \xrightarrow
of the AMSMath package: \xlongequal, \xLongleftarrow,
\xLongrightarrow, \xLongleftrightarrow, \xLeftrightarrow.
\xlongleftrightarrow, \xleftrightarrow, \xlongleftarrow and
\xlongrightarrow.
