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)))