getVars (Equation t1 _ t2 _) = Map.unionWith Set.union v1 v2
     where v1 = getVarsTerm t1
           v2 = getVarsTerm t2
getVars (Equation t1 _ t2 _) = Map.unionWith Set.union v1 v2
     where v1 = getVarsTerm t1
           v2 = getVarsTerm t2
getVars (Equation t1 _ t2 _) = Map.unionWith Set.union v1 v2
     where v1 = getVarsTerm t1
           v2 = getVarsTerm t2