HTTPS/HTTP/FTP - different proxies

I'm using SQUID as a caching proxy behind a firewall. The
firewall contains a HTTP-Proxy, that scans for viruses. I want
to use SQUID to forward all HTTP request to that proxy.
The problem: That proxy cannot handle HTTPS request ! Thus,
I want to forward all HTTPS request to another proxy.
Third I want to use SQUID to forward all FTP requests to the
virus scanning proxy via HTTP (not using ftpget).

Any ideas how to configure squid using two different proxys for
https and ftp/http ?

Thanks

Daniel Matuschek

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