let remove_underscores s =
let l = String.length s in
let rec remove src dst =
if src >= l then
if dst >= l then s else String.sub s 0 dst
else
match s.[src] with
'_' -> remove (src + 1) dst
| c -> s.[dst] <- c; remove (src + 1) (dst + 1)
in remove 0 0