f(a b -- x y)
  echo "a=$a b=$b x=$x y=$y"

assert $(f 1 2)             is "a=1 b=2 x= y="
assert $(f 1 2 --x X)       is "a=1 b=2 x=X y="
assert $(f 1 2 --y Y)       is "a=1 b=2 x= y=Y"
assert $(f 1 2 --x X --y Y) is "a=1 b=2 x=X y=Y"
assert $(f     --x X --y Y) is "a= b= x=X y=Y"
assert $(f 2   --x X --y Y) is "a=2 b= x=X y=Y"

g(-- arc)
  echo $arc

assert $(g --arc 1) is 1
assert $(g -a 1)    is 1
assert $(g)         is ${}