git replace: should it check for object type, and can it replace merges?

A recent comment http://stackoverflow.com/a/18027030/717355 on a question I asked two years ago about 'grafts' and 'replace' indicates that users think that 'git replace' can't replace a merge commit. The documentation doesn't have any examples and gives the naive impression that one should only replace a simple commit with another simple commit.

Having looked at the code, I realised that anything can be replaced with anything, which is perhaps not what was intended. A simple thought is that the replace should be like-for-like with regard to object type, though that would not include replacing a sub-module for a tree (and vice versa).

Should 'git replace' check the object types to ensure they are sensible?

Would it be reasonable to add examples to indicate the range of replacements, and how to prepare alternative merge commits, or is that a hostage to fortune?

Philip Oakley