code: func (st *state) unnamedTypeName() AST {
  	st.checkChar('U')
  	st.checkChar('t')
  	num := st.compactNumber()
  	ret := &UnnamedType{Num: num}
  	st.subs.add(ret)
  	return ret
  }
docs: // <unnamed-type-name> ::= Ut [ <nonnegative number> ] _