The relation “is subset of” on the power set P(A) of a set A is
 

The relation is not symmetric, because $\mathrm{A}\subset \mathrm{B}$ does not imply that $\mathrm{B}\subset \mathrm{A}.$ But it is anti-symmetric because $\mathrm{A}\subset \mathrm{B} \mathrm{and} \mathrm{B}\subset \mathrm{A}\Rightarrow \mathrm{A}=\mathrm{B}$