ssl_add_certificate_key(+SSL0, 
+Certificate, +Key, -SSL)Add an additional certificate/key pair to SSL0, yielding SSL.
Certificate and Key are either strings or atoms 
that hold the PEM-encoded certificate plus certificate chain and private 
key, respectively. Using strings is preferred for security reasons.
This predicate allows dual-stack RSA and ECDSA servers (for example), 
and is an alternative for using the
certificate_key_pairs/1 option. As of 
OpenSSL 1.0.2, multiple certificate types with completely independent 
certificate chains are supported. If a certificate of the same type is 
added repeatedly to a context, the result is undefined. Currently, up to 
12 additional certificates of different types are admissible.