func (u *Union) String() string   { return TypeString(u, nil) }

// A Term represents a term in a Union.
type Term term

// NewTerm returns a new union term.
func NewTerm(tilde bool, typ Type) *Term { return &Term{tilde, typ} }

func (t *Term) Tilde() bool    { return t.tilde }
func (t *Term) Type() Type     { return t.typ }
func (t *Term) String() string { return (*term)(t).String() }

func (u *Union) String() string   { return TypeString(u, nil) }

// A Term represents a term in a [Union].
type Term term

// NewTerm returns a new union term.
func NewTerm(tilde bool, typ Type) *Term { return &Term{tilde, typ} }

func (t *Term) Tilde() bool    { return t.tilde }
func (t *Term) Type() Type     { return t.typ }
func (t *Term) String() string { return (*term)(t).String() }

	//  T ⊆ ~t == true
	//  T ⊆  T == true
	return !x.tilde || y.tilde
}

// disjoint reports whether x ∩ y == ∅.
// x.typ and y.typ must not be nil.
func (x *term) disjoint(y *term) bool {
	if debug && (x.typ == nil || y.typ == nil) {
		panic("invalid argument(s)")
	}
	ux := x.typ
	if y.tilde {
		ux = under(ux)
	}
	uy := y.typ
	if x.tilde {
		uy = under(uy)
	}
	return !Identical(ux, uy)

	//  T ⊆ ~t == true
	//  T ⊆  T == true
	return !x.tilde || y.tilde
}

// disjoint reports whether x ∩ y == ∅.
// x.typ and y.typ must not be nil.
func (x *term) disjoint(y *term) bool {
	if debug && (x.typ == nil || y.typ == nil) {
		panic("invalid argument(s)")
	}
	ux := x.typ
	if y.tilde {
		ux = under(ux)
	}
	uy := y.typ
	if x.tilde {
		uy = under(uy)
	}
	return !types.Identical(ux, uy)

// term has a tilde. In all other cases coreTerm returns (nil, false).
func coreTerm(tpar *TypeParam) (*term, bool) {
	n := 0
	var single *term // valid if n == 1
	var tilde bool
	tpar.is(func(t *term) bool {
		if t == nil {
			assert(n == 0)
			return false // no terms
		}
		n++
		single = t
		if t.tilde {
			tilde = true
		}
		return true

// term has a tilde. In all other cases coreTerm returns (nil, false).
func coreTerm(tpar *TypeParam) (*term, bool) {
	n := 0
	var single *term // valid if n == 1
	var tilde bool
	tpar.is(func(t *term) bool {
		if t == nil {
			assert(n == 0)
			return false // no terms
		}
		n++
		single = t
		if t.tilde {
			tilde = true
		}
		return true

	INTERFACE
	MAP
	PACKAGE
	RANGE
	RETURN

	SELECT
	STRUCT
	SWITCH
	TYPE
	VAR
	keyword_end

	additional_beg
	// additional tokens, handled in an ad-hoc manner
	TILDE
	additional_end
)

var tokens = [...]string{
	ILLEGAL: "ILLEGAL",

	EOF:     "EOF",
	COMMENT: "COMMENT",

	IDENT:  "IDENT",
	INT:    "INT",
	FLOAT:  "FLOAT",
	IMAG:   "IMAG",

					continue
				}
				terms = termlist{{t.Tilde(), t.Type()}}
			}
			tset.terms = tset.terms.union(terms)
			if len(tset.terms) > maxTermCount {
				return nil, fmt.Errorf("exceeded max term count %d", maxTermCount)
			}
		}
	case *types.TypeParam:
		panic("unreachable")
	default:
		// For all other types, the term set is just a single non-tilde term
		// holding the type itself.

		if err != nil {
			return nil // invalid type
		}
		for _, term := range terms {
			subpath := lockPath(tpkg, term.Type(), seen)
			if len(subpath) > 0 {
				if term.Tilde() {
					// Prepend a tilde to our lock path entry to clarify the resulting
					// diagnostic message. Consider the following example:
					//
					//  func _[Mutex interface{ ~sync.Mutex; M() }](m Mutex) {}
					//

)

// Single-expression type parameter lists and those that don't start
// with a (type parameter) name are considered array sizes.
// The term must be a valid expression (it could be a type incl. a
// tilde term) but the type-checker will complain.
type (
        _[t] t
        _[t|t] t

        // These are invalid and the type-checker will complain.
        _[~t] t
        _[~t|t] t
        _[t|~t] t