style update for some string functions

src/core/CCString.cppo.ml

@@ -492,18 +492,20 @@ let repeat s n =
 
 let prefix ~pre s =
   String.length pre <= String.length s &&
-  (let i = ref 0 in
+  begin
+    let i = ref 0 in
     while !i < String.length pre && s.[!i] = pre.[!i] do incr i done;
     !i = String.length pre
-  )
+  end
 
 let suffix ~suf s =
   String.length suf <= String.length s &&
+  begin
     let off = String.length s - String.length suf in
-  (let i = ref 0 in
+    let i = ref 0 in
     while !i < String.length suf && s.[off + !i] = suf.[!i] do incr i done;
     !i = String.length suf
-  )
+  end
 
 let take n s =
   if n < String.length s

src/core/CCString.mli

@@ -212,6 +212,10 @@ val prefix : pre:string -> string -> bool
   prefix ~pre:"aab" "aabcd"
   not (prefix ~pre:"ab" "aabcd")
   not (prefix ~pre:"abcd" "abc")
+  prefix ~pre:"abc" "abcde"
+  prefix ~pre:"" ""
+  prefix ~pre:"" "abc"
+  prefix ~pre:"abc" "abc"
 *)
 
 val suffix : suf:string -> string -> bool
@@ -220,6 +224,8 @@ val suffix : suf:string -> string -> bool
 
 (*$T
   suffix ~suf:"cd" "abcd"
+  suffix ~suf:"" ""
+  suffix ~suf:"" "abc"
   not (suffix ~suf:"cd" "abcde")
   not (suffix ~suf:"abcd" "cd")
 *)