gistfile1.sh

function f {
  local s='old'
  echo -n "${(r:15:: :)1}${(e)1}"
  s=''
  echo -n "\t${(e)1}"
  unset s
  echo -n "\t${(e)1}"
  echo
}

f '${s}'
f '${+s}'
f '${s-new}'
f '${s:-new}'
f '${s+new}'
f '${s:+new}'
f '${s=new}'
f '${s:=new}'
f '${s::=new}'


# ${s}           old
# ${+s}          1        1       0
# ${s-new}       old              new
# ${s:-new}      old      new     new
# ${s+new}       new      new
# ${s:+new}      new
# ${s=new}       old              new
# ${s:=new}      old      new     new
# ${s::=new}     new      new     new