is_eq `t1 = t2` returns true. If the term is not an equation the result
is false. Note that logical equivalence is just equality on type :bool,
even though it is printed as <=>.
FAILURE CONDITIONS
Never fails.
EXAMPLE
# is_eq `2 + 2 = 4`;;
val it : bool = true
# is_eq `p /\ q <=> q /\ p`;;
val it : bool = true
# is_eq `p ==> p`;;
val it : bool = false