homomorphy_form (Equation t1 e t2 r) = Equation t1' e t2' r
     where t1' = homomorphy_term t1
           t2' = homomorphy_term t2
homomorphy_form (Equation t1 e t2 r) = Equation t1' e t2' r
     where t1' = homomorphy_term t1
           t2' = homomorphy_term t2
homomorphy_form (Equation t1 e t2 r) = Equation t1' e t2' r
     where t1' = homomorphy_term t1
           t2' = homomorphy_term t2