Can't map alt_r to space, but I can map space to alt_r

[viasux]

# Map an unused modifier's keysym to the spacebar's keycode and make it a
# control modifier. It needs to be an existing key so that emacs won't
# spazz out when you press it. Hyper_L is a good candidate.
spare_modifier="space"
xmodmap -e "keycode 65 = $spare_modifier"
xmodmap -e "remove mod4 = $spare_modifier" # hyper_l is mod4 by default
xmodmap -e "add Control = $spare_modifier"

# Map space to an unused keycode (to keep it around for xcape to
# use).
xmodmap -e "keycode 255 = Alt_r"

# Finally use xcape to cause the space bar to generate a space when tapped.
# xcape -e 'Alt_R=KP_Space'
xcape -e "$spare_modifier=Alt_r"

gives this

xmodmap:  commandline:1:  bad keysym name 'Alt_r' in keysym list
xmodmap:  1 error encountered, aborting.
Invalid key: Alt_r
Failed to parse_mapping