A relation $\sim$ on $M$ is called 1) reflexive, if for any $a \in M$, we have $a \sim a$; 2) symmetric, if for any $a,b \in M$, we have $a \sim b \Rightarrow b \sim a$; 3) transitive, if for any $a,b,c \in M$ with $a \sim b$ and $b \sim c$, we have $a \sim c$.