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kotlinlang

I think the new contract `blah implies returnsNotNull()` is completely unnecessary. One can always rewrite it as the contrapositive `returns(null) implies !blah` . Is adding it really necessary then? Maybe it should stay in, but be made non-expiremental and simply add the same flags that the contrapositive would add.
In fact, the current restriction that only `returnsNotNull()` may show up on the RHS of `implies` is entirely pointless as well, and in fact rather confusing. It should also support `returns(null)` et all.

The only one that wouldn't make sense is `implies returns()` since that contract cannot be trusted (exceptions can always happen), unless you take it to mean something related to the purity of the function (which would be kinda fun, actually).