#once #create message
.ND
.tr %$
.EQ
delim $$
.EN
.LP
The way to handle the problem of a subscript or superscript
on nothing is to provide something to hang the subscript
or superscript on. 
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:

  "" sup 1 ...

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,
then type "ready".
.pl 1
#once #create Ref
.LP
.EQ
"" sup + H sub 2 ~ "<=>" ~ "" sup - H ~+~ "" sup + H
.EN
.pl 1
#once #create Example
.LP
.EQ
xxx
.EN
.pl 1
#
#once neqn Ref | nroff >X1 &
#once neqn message Ref | nroff -T$term %s/tinyms -
#user
neqn Example | nroff >X2
#cmp X1 X2
#fail
Don't forget spaces; don't forget to quote
things when needed (including <=>).
#log
#next
11.1g 10