Add `isless` function for `Variation`

src/Variation.jl

@@ -75,6 +75,11 @@ BioGenerics.leftposition(v::Variation) = leftposition(_edit(v))
 BioGenerics.rightposition(v::Variation) = rightposition(_edit(v))
 Base.:(==)(x::Variation, y::Variation) = x.ref == y.ref && x.edit == y.edit
 Base.hash(x::Variation, h::UInt) = hash(Variation, hash((x.ref, x.edit), h))
+function Base.isless(x::Variation, y::Variation)
+    reference(x) == reference(y) ||
+        error("Variations cannot be compared if their reference sequences aren't equal")
+    return leftposition(x) < leftposition(y)
+end
 
 """
     _is_valid(v::Variation)