covariant and contravariant rules

Subject: covariant and contravariant rules
From: Mayur Naik (naik@india.hp.com)
Date: Tue Dec 12 2000 - 04:26:48 PST

Hello

I came across the terms 'covariant rule' and contravariant rule' in the
paper 'Typechecking and Modules for Multi-Methods.' Can you please tell
me what these rules are about or give some reference?

Thanks.

        -- Mayur

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