func (s *_TypeSet) underIs(f func(Type) bool) bool {
	if !s.hasTerms() {
		return f(nil)
	}
	for _, t := range s.terms {
		assert(t.typ != nil)
		// x == under(x) for ~x terms
		u := t.typ
		if !t.tilde {
			u = under(u)
		}
		if debug {
			assert(Identical(u, under(u)))
		}
		if !f(u) {
			return false
		}
	}
	return true
}

func (s *_TypeSet) underIs(f func(Type) bool) bool {
	if !s.hasTerms() {
		return f(nil)
	}
	for _, t := range s.terms {
		assert(t.typ != nil)
		// x == under(x) for ~x terms
		u := t.typ
		if !t.tilde {
			u = under(u)
		}
		if debug {
			assert(Identical(u, under(u)))
		}
		if !f(u) {
			return false
		}
	}
	return true
}

			}
		// Unreserved according to RFC 3986 sec 2.3
		// "For consistency, percent-encoded octets in the ranges of
		// ALPHA (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D),
		// period (%2E), underscore (%5F), or tilde (%7E) should not be
		// created by URI producers
		case '-', '.', '_', '~':
			continue
		case '%':
			// When normalizing do not re-encode valid escapes.

1E4D          ; valid                                  # 1.1  LATIN SMALL LETTER O WITH TILDE AND ACUTE
1E4E          ; mapped                 ; 1E4F          # 1.1  LATIN CAPITAL LETTER O WITH TILDE AND DIAERESIS
1E4F          ; valid                                  # 1.1  LATIN SMALL LETTER O WITH TILDE AND DIAERESIS

		x = t
	}
	return x
}

func (p *parser) embeddedTerm() ast.Expr {
	if p.trace {
		defer un(trace(p, "EmbeddedTerm"))
	}
	if p.tok == token.TILDE {
		t := new(ast.UnaryExpr)
		t.OpPos = p.pos
		t.Op = token.TILDE
		p.next()
		t.X = p.parseType()
		return t
	}

	t := p.tryIdentOrType()
	if t == nil {
		pos := p.pos
		p.errorExpected(pos, "~ term or type")

  pymdownx.superfences:
    custom_fences:
    - name: mermaid
      class: mermaid
      format: !!python/name:pymdownx.superfences.fence_code_format ''
  pymdownx.tabbed:
    alternate_style: true
  pymdownx.tilde: null
  attr_list: null
  md_in_html: null
extra:
  analytics:
    provider: google
    property: G-YNEVN69SC3
  social:
  - icon: fontawesome/brands/github-alt

	case *ast.Ident:
		// nothing to do
	case *ast.ParenExpr:
		r.filterType(nil, t.X)
	case *ast.StarExpr: // possibly an embedded type literal
		r.filterType(nil, t.X)
	case *ast.UnaryExpr:
		if t.Op == token.TILDE { // approximation element
			r.filterType(nil, t.X)
		}
	case *ast.BinaryExpr:
		if t.Op == token.OR { // union
			r.filterType(nil, t.X)
			r.filterType(nil, t.Y)
		}
	case *ast.ArrayType:

}

// EmbeddedTerm = [ "~" ] Type .
func (p *parser) embeddedTerm() Expr {
	if trace {
		defer p.trace("embeddedTerm")()
	}

	if p.tok == _Operator && p.op == Tilde {
		t := new(Operation)
		t.pos = p.pos()
		t.Op = Tilde
		p.next()
		t.X = p.type_()
		return t
	}

	t := p.typeOrNil()
	if t == nil {
		t = p.badExpr()
		p.syntaxError("expected ~ term or type")

		// parameters.
		reportReason := len(m.seen) == 1

		for _, term := range terms {
			if !m.match(term.Type(), topLevel) {
				if reportReason {
					if term.Tilde() {
						m.reason = fmt.Sprintf("contains ~%s", term.Type())
					} else {
						m.reason = fmt.Sprintf("contains %s", term.Type())
					}
				}
				return false
			}
		}
		return true
	}

			// representable by each specific type in the type set of T.
			return Tp.is(func(t *term) bool {
				if t == nil {
					return false
				}
				// A term may be a tilde term but the underlying
				// type of an untyped value doesn't change so we
				// don't need to do anything special.
				newType, _, _ := check.implicitTypeAndValue(x, t.typ)
				return newType != nil