The way to handle the problem of a subscript or superscript
on nothing is to provide something to hang the subscript
One possibility is to use a tilde, but that has a real width,
and might spoil the appearance of the output.
The best solution is this:
The expression "" has no width, so everything works
out properly. The use of "" may seem a bit unnatural
but it's an idiom that you get used to quite quickly.
For practice, modify "Example" so it produces
output that looks like this,
"" sup + H sub 2 ~ "<=>" ~ "" sup - H ~+~ "" sup + H
#once neqn Ref | nroff >X1 &
#once neqn message Ref | nroff -T$term %s/tinyms -
Don't forget spaces; don't forget to quote
things when needed (including <=>).