code: func (st *state) taggedName(a AST) AST {
  	for len(st.str) > 0 && st.str[0] == 'B' {
  		st.advance(1)
  		tag := st.sourceName()
  		a = &TaggedName{Name: a, Tag: tag}
  	}
  	return a
  }
docs: // <tagged-name> ::= <name> B <source-name>