let check_form s f =
match f with
Suffix sfx ->
let frmlen = String.length sfx in
let slen = String.length s in
slen >= frmlen &&
String.compare (String.sub s (slen - frmlen) frmlen) sfx = 0
| Prefix pfx ->
let frmlen = String.length pfx in
String.length s >= frmlen &&
String.compare (String.sub s 0 frmlen) pfx = 0
| Exact ext ->
let frmlen = String.length ext in
String.length s = frmlen &&
String.compare s ext = 0