Module certificate 

Structs

DoubleCertificate

C_v(C_v(B_k)): double certificate — QC of QC, triggers commit

QuorumCertificate

C_v(B_k): quorum certificate — 2f+1 validators signed the block hash in view v

TimeoutCertificate

TC_v: timeout certificate — 2f+1 validators want to leave view v