span.test

<<< tex
Y \xleftarrow{f} X \xrightarrow{g} Z


>>> native
[ EIdentifier "Y"
, EOver False (ESymbol Op "\8592") (EIdentifier "f")
, EIdentifier "X"
, EOver False (ESymbol Op "\8594") (EIdentifier "g")
, EIdentifier "Z"
]