comparable bool     // invariant: !comparable || terms.isAll()
}

// IsEmpty reports whether type set s is the empty set.
func (s *_TypeSet) IsEmpty() bool { return s.terms.isEmpty() }

// IsAll reports whether type set s is the set of all types (corresponding to the empty interface).
func (s *_TypeSet) IsAll() bool { return s.IsMethodSet() && len(s.methods) == 0 }

	comparable bool     // invariant: !comparable || terms.isAll()
}

// IsEmpty reports whether type set s is the empty set.
func (s *_TypeSet) IsEmpty() bool { return s.terms.isEmpty() }

// IsAll reports whether type set s is the set of all types (corresponding to the empty interface).
func (s *_TypeSet) IsAll() bool { return s.IsMethodSet() && len(s.methods) == 0 }

	// one iteration.
	for _, x := range xl {
		if x != nil {
			return false
		}
	}
	return true
}

// isAll reports whether the termlist xl represents the set of all types.
func (xl termlist) isAll() bool {
	// If there's a 𝓤 term, the entire list is 𝓤.
	// If the termlist is in normal form, this requires at most
	// one iteration.
	for _, x := range xl {

	// one iteration.
	for _, x := range xl {
		if x != nil {
			return false
		}
	}
	return true
}

// isAll reports whether the termlist xl represents the set of all types.
func (xl termlist) isAll() bool {
	// If there's a 𝓤 term, the entire list is 𝓤.
	// If the termlist is in normal form, this requires at most
	// one iteration.
	for _, x := range xl {

		"~int | myInt":  false,
		"∅ | ∅ | 𝓤":     true,
		"𝓤":             true,
		"𝓤 | int":       true,
		"myInt | 𝓤":     true,
	} {
		xl := maketl(test)
		got := xl.isAll()
		if got != want {
			t.Errorf("(%v).isAll() == %v; want %v", test, got, want)
		}
	}
}

func TestTermlistNorm(t *testing.T) {
	for _, test := range []struct {
		xl, want string
	}{
		{"∅", "∅"},
		{"∅ | ∅", "∅"},

		"~int | myInt":  false,
		"∅ | ∅ | 𝓤":     true,
		"𝓤":             true,
		"𝓤 | int":       true,
		"myInt | 𝓤":     true,
	} {
		xl := maketl(test)
		got := xl.isAll()
		if got != want {
			t.Errorf("(%v).isAll() == %v; want %v", test, got, want)
		}
	}
}

func TestTermlistNorm(t *testing.T) {
	for _, test := range []struct {
		xl, want string
	}{
		{"∅", "∅"},
		{"∅ | ∅", "∅"},

	// one iteration.
	for _, x := range xl {
		if x != nil {
			return false
		}
	}
	return true
}

// isAll reports whether the termlist xl represents the set of all types.
func (xl termlist) isAll() bool {
	// If there's a 𝓤 term, the entire list is 𝓤.
	// If the termlist is in normal form, this requires at most
	// one iteration.
	for _, x := range xl {

// The methods are ordered by their unique Id.
func (t *Interface) Method(i int) *Func { return t.typeSet().Method(i) }

// Empty reports whether t is the empty interface.
func (t *Interface) Empty() bool { return t.typeSet().IsAll() }

// IsComparable reports whether each type in interface t's type set is comparable.
func (t *Interface) IsComparable() bool { return t.typeSet().IsComparable(nil) }

	tset, err := computeTermSetInternal(typ, make(map[types.Type]*termSet), 0)
	if err != nil {
		return nil, err
	}
	if tset.terms.isEmpty() {
		return nil, ErrEmptyTypeSet
	}
	if tset.terms.isAll() {
		return nil, nil
	}
	var terms []*types.Term
	for _, term := range tset.terms {
		terms = append(terms, types.NewTerm(term.tilde, term.typ))
	}
	return terms, nil
}

// The methods are ordered by their unique Id.
func (t *Interface) Method(i int) *Func { return t.typeSet().Method(i) }

// Empty reports whether t is the empty interface.
func (t *Interface) Empty() bool { return t.typeSet().IsAll() }

// IsComparable reports whether each type in interface t's type set is comparable.
func (t *Interface) IsComparable() bool { return t.typeSet().IsComparable(nil) }