String comparison by collation as described in UTR #10
How variables are handled
Strength of comparison. For European languages, each strength roughly means as `Primary : Ignore accents and case `Secondary : Ignore case but accents are counted in. `Tertiary : Accents and case are counted in. For the case of `Shifted, `Shift_Trimmed, there is the fourth strength. `Quaternary : Variables such as - (hyphen) are counted in.
module type Type = sig ... end