Fix an implementation/specification mismatch in BatVect.insert

[gasche]

BatVect.insert : int -> 'a t -> 'a t is specified so that insert n r u inserts the rope r between the elements of index n and n+1 in u. Its implementation does something different, it inserts r strictly before the element of index n in u.

The present commit changes the specification to match the implementation.

First, we expect users to have tested their program instead of trusting (or, unfortunately, even reading) the documentation, so it is likely that the uses of BatVect.insert with the current implementation is correct for their needs; changing it would break user program.

Second, we argue that the implemented behavior is actually far more
natural:

• the implementation (in terms of sub and concat) is nicer -- hear the code!

• the ranges of integer values valid for insert is 0 .. (length u - 1), as one would expect, instead of (-1) .. (length u - 2) as with the current specification.

• this gives a very natural invariant between insert and BatVect.remove : (*start*)int -> (*len*)int -> 'a t -> 'a t: u is equal to u |> insert n r |> remove n (length r)).

fixes #766.

[UnixJunkie]

I get this when I do 'make test':

File "src/batArray.ml", line 179, characters 40-51:
Error: This expression has type int Q.arbitrary = int QCheck.arbitrary
       but an expression was expected of type
	    'a Q.Observable.t = 'a QCheck.Observable.t

I am using qtest.2.5.

[c-cube]

You probably need to upgrade to qcheck 0.6, OR to update the test :) The function generators have been updated (in the good direction, I like to think: no more magic, shrinking on functions, etc). -- Simon

[gasche]

Currently the tests in Batteries require (qtest >= 2.0 & qtest < 2.5), see the opam file.