demangle.go
func (st *state) checkChar(c byte) {
	if len(st.str) == 0 || st.str[0] != c {
		panic("internal error")
	}
	st.advance(1)
}
func (st *state) checkChar(c byte) {
	if len(st.str) == 0 || st.str[0] != c {
		panic("internal error")
	}
	st.advance(1)
}
// checkChar requires that the next character in the string be c, and
// advances past it.