[Zope] How to assing a string value to a variable in DTML?

[iap_y2fun.com]

Forgive me for this stupid question:
How to assing a string value to a variable in DTML?
for example:

<dtml-let a='aaaaa'
b='bbbbb'>
<dtml-var "a+b">
</dtml-let>

it causes error, not what I expected:
given a="aaaa", b="bbbb", and eval a+b to "aaaabbbb"

Thanks

iap