using (in-readtable) to select a readtable

[73budden .]

> 2. Create variable, *lock-readtable-alist*. If it is true, error is
> signalled on attempt to alter *readtable-alist*
I mean error is signalled when (in-readtable) attempts to alter
*readtable-alist*.
Manual altering would be still allowed.