Does this need another unicode.SpecialCase in https://golang.org/pkg/strings/#ToLowerSpecial ?

I do see a TODO in unicode/casetables.go.

@robpike @ianlancetaylor

CC @mpvl

Unicode case folding requires handling the final sigma special case, but the rule is overridden in a few standards; for example Appendix C of rfc7790 (PRECIS) says:

local case mapping is not applicable to small sigma or final sigma, so case mapping in the PRECIS framework always maps final sigma to small sigma, independent of context

Changing the strings.ToLower function to handle the final sigma (in full compliance with Unicode Folding rules) may break existing code relying on the current behaviour. Also from a cursory look (but I may be wrong) the current special-case mechanism in unicode does not support context-sensitive replacement rules, so it may be not trivial to implement the rule in a non-hacky way.

On the other hand, the text/cases package handles the final sigma special case, and also provides a way to get a PRECIS compliant folding:

package main

import (
	"fmt"

	"golang.org/x/text/cases"
	"golang.org/x/text/language"
)

func main() {
	greekLower1 := cases.Lower(language.Greek)
	greekLower2 := cases.Lower(language.Greek, cases.HandleFinalSigma(false))

	fmt.Println(greekLower1.String("β︎Δℕ︎Σ"))   // prints β︎δℕ︎ς
	fmt.Println(greekLower2.String("β︎Δℕ︎Σ"))   // prints β︎δℕ︎σ
}

My proposal is to preserve the existing strings behaviour, and maybe add a small note about the final sigma handling in the documentation, and to point users to the text/cases package for full Unicode Compliant folding.