# If-and-only-if

If-and-only-if expresses biconditional logical connective between two statements. The statements should contain X/Y variables. It is used for making axioms on properties/roles (such as **have-child** or **have-sibling**).

### Examples

Those examples show how make axioms on properties

Example: has-child is reverse of has-parent.

X has-child Y if-and-only-if Y has-parent X.

Example: has-sibling is symmetric.

X has-sibling Y if-and-only-if Y has-sibling X.