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type t = char
let minus = '-'
let plus = '+'
let of_string s = match String.lowercase s with
| "-" | "rev" -> Ok minus
| "+" | "fwd" -> Ok plus
| _ -> error "unknown strand name" s sexp_of_string
let minus_plus = Fn.id
let rev_fwd = function
| '-' -> "rev"
| '+' -> "fwd"
| _ -> assert false