Verify signatures other than first
Propose adding an idx= parameter to verify method, and other methods to verify all, and make information about verified signatures available (array of sig dicts?).
Test cases and multiple signatures:
The special treatment for DKIM-Signature applies only to the one about to be inserted. Already existing DKIM-Signature fields are signed like any other header field. The DKIM-Signature about to be inserted is not reflected in h= (rfc 4871 5.4):
The DKIM-Signature header field is always implicitly signed and MUST
NOT be included in the "h=" tag except to indicate that other
pre-existing signatures are also signed.
While this could be clearer, it seems clear to me that pre-existing DKIM-Signature fields are signed just like any other header. In particular, there is this logical consequence: including dkim-signature in h= when there is no pre-existing dkim-signature (or one more than the number of pre-existing dkim-signatures) effectively prohibits additional signatures (or the signature you are adding won't verify). It does not refer to the dkim-signature about to be inserted.
How do we check for this in a test? We could include properly signed messages with dkim-signature in h= as test cases. Another test case we can do is to sign a message with existing dkim-signature (including the existing signature). Changing the b= should cause the signature to fail.