set, dict and other structures

[bearophileHUGS at lycos.com]

r = {}
for x in a: if x not in b: r[x] = r[a]
for x in b: if x not in a: r[x] = r[b]

I know, this is wrong :-]
This looks better:
r = {}
for x in a: if x not in b: r[x] = a[x]
for x in b: if x not in a: r[x] = b[x]

Bearophile