Relation R is symmetric if R(x,y) implies R(y,x).
(=> (Holds ?R ?X ?Y) (Holds ?R ?Y ?X))
(<=> (Symmetric-Relation ?R) (And (Binary-Relation ?R) (=> (Holds ?R ?X ?Y) (Holds ?R ?Y ?X))))
(Binary-Relation ?R)
(<=> (Symmetric-Relation ?R) (And (Binary-Relation ?R) (=> (Holds ?R ?X ?Y) (Holds ?R ?Y ?X)))) (<=> (Equivalence-Relation ?R) (And (Reflexive-Relation ?R) (Symmetric-Relation ?R) (Transitive-Relation ?R)))