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