defzza{}
defzzb{a}
defzzc{defzze{}}
defzzd{ifmmode a else bfi}

edefyyb{zzb}

edefyyc{zzc}

edefyyc{defzze{}}

protecteddefzzc{defzze{}}

edefyyc{zzc}

edefyyd{zzd}

edefyyd{ifmmode a else bfi}

defyyd{b}

deftesta#1#2#3{%
  ifnum#1=0
  defnext{#2}%
  else
  defnext{#3}%
  fi
  next}

deffirstoftwo#1#2{#1}
defsecondoftwo#1#2{#2}

deftestb#1{%
  ifnum#1=0
  expandafterfirstoftwo
  else
  expandaftersecondoftwo
  fi}

deftestc#1#2{%
  deftempa{#1}deftempb{#2}%
  ifxtempatempb
  expandafterfirstoftwo
  else
  expandaftersecondoftwo
  fi}

newdimmydim
mydimwdhbox{foo}%

newdimmydim
newboxmybox
setboxmybox=hbox{foo}%
mydimwdmybox

foo{[} ...