The way autocorrect is implemented it is not possible to map
--> and ==> to \longleftarrow and \Longleftarrow, respectively.
One has to type ->- and =>= to that effect, which may be
counterintuitive.
This commit allows typing -> or => followed by * to cycle through
all possible arrows of the same type.
\Leftarrow = \Longleftarrow
\Longleftarrow > \Longleftrightarrow
+\leftarrow * \longleftarrow
+\longleftarrow * \longleftrightarrow
+\longleftrightarrow * \leftarrow
+\rightarrow * \longrightarrow
+\longrightarrow * \longleftrightarrow
+
+\Leftarrow * \Longleftarrow
+\Longleftarrow * \Longleftrightarrow
+\Longleftrightarrow * \Leftarrow
+\Rightarrow * \Longrightarrow
+\Longrightarrow * \Longleftrightarrow
@ * \circ
\circ / \varnothing